Lecture 13#

Reading: H13.1

Contents:-Network Administration: Introduction to Network Administration Approaches

Network administration means the management of network infrastructure devices (routers and switches). Network administration is the management of PCs in a network. Goal of network administration:-

The goal of network administration is to ensures that the users of networks receive the information and technically serves with quality of services they except.

Network administration means the management of network infrastructures devices (such as router and switches)

Network administration compromises of 3 majors groups:1. Network provisioning2. Network operations3. Network maintenance

Network provisioning: - is the primary responsibility of engineering groups and its consists of planning and design of network which is done by engineer.

Network operations: - it consists of fault, configurations, traffic, all type of management and it is done by plant facilities group. Its is nerve center of network management operations.

Network maintenance:- its consists of all type of installations and maintenance work.

Lecture no 14#Reading:Content:Addressing and Subnetting: Fixed Vs Variable Masks

Internet Architecture and IP Addresses

Introduction of TCP/IP Internet

Internet Architecture Physical network: computers on the same physical network are physically connected. Computers on different physical networks are not physically connected. IP router (or IP gateway): dedicated systems that connect two or more networks. Host: end-user system. It connects to physical networks, and there are possibly many

hosts per network

The two view of a TCP/IP Internet

Packet Transmission Source Host:

If the destination is on the same physical network, deliver it directly Otherwise, send it to a router

Intermediate Routers: The destination is not on the same physical network, forward the packet to another

router Final Router

The destination is physically connected to this final router, so send the packet directly to the destination.

How do routers work?

Routers need to find the right routes when forwarding packets. Routers’ decision is based on the routing information they have Routing table: use destination network, not the destination host; otherwise, the table

will be huge.

IP Address Overview It is basically an identifier used in the network layer of the internet model to identify each

device is connected to internet or not. 32 bit binary value Unique value assigned to each host Values chosen to make routing efficient Dotted Decimal Notation:

Binary: 10000000 11100110 00000001 00001100 Dotted decimal notation: 128.230.1.12

The IP address consists of a pair of numbers:IP address = <network number><host number>The network number portion of the IP address is administered by one of threeRegional Internet Registries (RIR):

American Registry for Internet Numbers (ARIN): This registry is responsible for the administration and registration of Internet Protocol (IP) numbers for North America, South America, the Caribbean, and sub-Saharan Africa.

Reseaux IP Europeans (RIPE): This registry is responsible for the administration and registration of Internet Protocol (IP) numbers for Europe, Middle East, and parts of Africa.

Asia Pacific Network Information Centre (APNIC): This registry is responsible for the administration and registration of Internet Protocol (IP) numbers within the Asia Pacific region.

The division of an IP address into two parts also separates the responsibility for selecting the complete IP address. The network number portion of the address is assigned by the RIRs. The host number portion is assigned by the authority controlling the network. As shown in the next section, the host number can be further subdivided: This division is controlled by the authority that manages the network. It is not controlled by the RIRs.

Classful Addressing Scheme (The original scheme, didn’t last long) In this scheme the address space is divided into 5 classes as shown below

Above class support unicast address schemes.

Properties of the classful addressing scheme?

They are self-identifying: the boundary between netid and hostid is self-explained from the address. This can benefit routing because the entries of routing tables store mainly use netid, not the entire IP address.

Class A:- Range (1-126) 1st bit is always 0 Range of network no’s 1.0.0.0 --- 126.0.0.0 No of possible network is 127 and out of this 1-126 is used & 127 & 0 is not used. No of possible values in host portions is 16,777,216 (256*256*256 ) Advantages:- used for large network Disadvantages: - millions of class A address are wasted.

Class B:- Range (128-191) Always 1st two bits is 10 Range of network no’s 128.0.0.0 --- 191.255.0.0 No’s of possible networks 16,384 blocks (64*256) No of possible values in host portions is 65,536(256*256) Advantages: - used for midsize network Disadvantages: - many of class B address are wasted.

Class C:- Range (192-223) Always 1st three bits is 110 Range of network no’s 192.0.0.0 --- 223.255.255.0 No’s of possible networks 2,097,152 blocks (32*256*256) No of possible values in host portions is 256 Advantages: - used for small network Disadvantages: - no’s of address in class C is smaller than the needs of mosts organizations.

Class D :- Range (224-239) Always 1st Four bits is 1110 Range of network no’s 224.0.0.0 --- 239.255.255.255 It is used for multicast.

Class E :- Range (240-255) Always 1st Five bits is 11110 Range of network no’s 240.0.0.0 --- 255.255.255.254 It is used for research purpose.

PROBLEMS OF CLASSFUL ADDRESSING SCHEMES:-

In classful addressing schemes each class is divided into a fixed no of blocks where each blocks have affixed size

CLASS A:-Total 128 blocks1st block -0.0.0.0---------0.255.255.255Last block 127.0.0.0-----------127.255.255.255Private address range 10.0.0.0-------10.0.0.255 (1 block)so total no of block used is 125.So main disadvantages is that million of class A address are wasted because no of address in each blocks is 16,777,216

Class BTotal blocks 16384(out of that we used only 16,368)Each blocks contains address -65,53616 blocks are reserved for private addressing.Range 172.16.0.0 ---------172.31.255.255

Class CTotal blocks 2,097,152 (out of that we use 2,096,896)256 blocks are used for private addressing.Private address range 192.168.0.0 ---------192.168.255.256Each block contains 256 addresses.

Class DIt contain 1 blockUsed for multicasting Class EIts contain 1 blockUsed for reserved address.

Reserved address prefixes

a) 10/8 10.0.0.0 - 10.255.255.255b) 172.16/12 172.16.0.0 - 172.31.255.255c) 192.168/16 192.168.0.0 - 192.168.255.255d) 169.254/16 169.254.0.0 - 169.254.255.255

Special Addresses

255.255.255.255: Limited broadcast (local net) 0.0.0.0: this host. Can only be used as source address. It is used during bootstrap before a

computer knows its IP address. “0” means THIS. net + all 1s: directed broadcast for net 127. Anything (often 1): loop back.

Reserved IP addressesA component of an IP address with a value all bits 0 or all bits 1 has a specialmeaning:

All bits 0: An address with all bits zero in the host number portion is interpreted as this host (IP address with <host address>=0). All bits zero in the network number portion is this network (IP address with <network address>=0). When a host wants to communicate over a network, but does not yet know the network IP address, it can send packets with <network address>=0. Other hosts in the network interpret the address as meaning this network. Their replies contain the fully qualified network address, which the sender records for future use.

All bits 1: An address with all bits one is interpreted as all networks or all hosts. For example, the following means all hosts on network 128.2 (Class B address):128.2.255.255 This is called a directed broadcast address because it contains 128.2.255.256 both a valid

<network address> and a broadcast <host address>. Loopback: The Class A network 127.0.0.0 is defined as the loopback network. Addresses from

that network are assigned to interfaces that process data within the local system. These loopback interfaces do not access a physical network.

Unicast address: - it’s used to communicate from one source to one destination.Multicast Address: - it’s used to communicate from one source to group of destination. & it can be used only as destination address only.Broadcast address: - communication is from one to all

Note: - address space is 2 power N

Where N is no of bits

An IP address are designed with 2 level hierarchy1) netID2) hostID

Network ID (netID): - the hosts that populates that networks shares those same bits called networks bits.Host ID:-these are unique identifier of each hosts within that network.

Network address:-a network address is an address that defines network itself it cannot be assigned to a host.

Property of network address:-1) all hosts ID bytes are 0’s2) The network address defines the networks to the rest of the internet.3) Network address is the 1st address in the blocks.4) If given an network address we can define class of address

NOTE: - A network address is different from netID. A network address has both netID& hostID with 0’s for hostID.

Eg. Given the address 23.56.7.91. Find the network address?Sol: - the class is A because first byte define netID. So we can find network address by replacing hostID bytes by 0’s so network address is 23.0.0.0

SUBNETTING

If you wanted to take one network address and create six networks from it? You would have to perform what is called subnetting, which allows you to take one larger network and break it into many smaller networks. There are many reasons to perform subnetting. Some of the benefits of subnetting include the following:Reduced network traffic We all appreciate less traffic of any kind. Networks are no different. Without trusty routers, packet traffic could grind the entire network down to a near standstill. With routers, most traffic will stay on the local network; only packets destined for other networks will pass through the router. Routers create broadcast domains. The smaller broadcast domains you create the less network traffic on that network segment.Optimized network performance This is a result of reduced network traffic.Simplified management It’s easier to identify and isolate network problems in a group of smaller connected networks than within one gigantic network.Facilitated spanning of large geographical distances Because WAN links are considerably slower and more expensive than LAN links, a single large network that spans long distances can create problems in every arena listed above. Connecting multiple smaller networks makes the system more efficient.

Subnet Masks

For the subnet address scheme to work, every machine on the network must know which part of the host address will be used as the subnet address. This is accomplished by assigning a subnet mask to each machine. This is a 32-bit value that allows the recipient of IP packets to distinguish the network ID portion of the IP address from the host ID portion of the IP address.

When router receives packet with destination IP address it needs to route a packets & the routing is based on the network address & subnetwork address so the router outside the organization routes the packets based on network address & router inside the organization route the packets based on subnetwork address.

ROUTER OUTSIDE = USES DEFAULT MASK

ROUTER INSIDE = USES SUBNET MASK

The network administrator creates a 32-bit subnet mask composed of 1s and 0s. The 1s in the subnet mask represent the positions that refer to the network or subnet addresses.

DEFAULT MASK

It’s a 32 bit binary no’s that gives 1s t address in the block (network address)When ANDed with address in the block.

Rules of masking:-1) If mask byte is255 retain corresponding byte.2) if mask byte is255 set corresponding byte to 0

Eg. Given following address and use default mask to find network address.1) 23.56.7.912) 132.16.17.853) 201.180.56.5

Sol 1) 23.56.7.91----- class A 255.0.0.0 --------default mask of class A 23.0.0.0 ---------- network address by using masking rule

2) 132.16.17.85----- class B 255.255.0.0 --------default mask of class B 132.16.0.0 ---------- network address by using masking rule

3) 201.180.56.5----- class C 255.255.255.0 --------default mask of class C 201.180.56.0 ---------- network address by using masking rule

Contiguous subnetmask

Are those 11110000 (no’s of 1 followed by 0’s)

Non Contiguous subnetmaskStrings with mixture of 0 & 1’s.

Notes: adding subnetting to ip address will create 3 level of hierarchya) siteb) subnetIDc) hosted

Classless Addressing Scheme (Devised in 1990s)

Allow the division between prefix and suffix to occur at an arbitrary point. Allow more complete utilization of the address space.

(2) CIDR: Classless Inter-Domain Routing

a) Internet Part + Local Partb) Internet Part + Physical Network + Hosti) Example: IP:128.230.211.195. Netmask FFFFF800ii) 128 = 1000 0000, 230 = 1110 0110, 211 = 11010011iii) What is the CIDR representation? What are the lowest IP and highest IP addresses?iv) Is Apollo (128.230.208.46) on the same subnet? 208 = 1101 0000

Lecture no # 15

Reading: - www.wikipedia.comContents:-VLAN Principles and Configuration

VLAN Principles and Configuration

What are VLAN’s?

In a traditional LAN, workstations are connected to each other by means of a hub or a repeater. These devices propagate any incoming data throughout the network. However, if two people attempt to send information at the same time, a collision will occur and all the transmitted data will be lost. Once the collision has occurred, it will continue to be propagated throughout the network by hubs and repeaters. The original information will therefore need to be resent after waiting for the collision to be resolved, thereby incurring a significant wastage of time and resources.

To prevent collisions from traveling through all the workstations in the network, a bridge or a switch can be used. These devices will not forward collisions, but will allow broadcasts (to every user in the network) and multicasts (to a pre-specified group of users) to pass through.

A router may be used to prevent broadcasts and multicasts from traveling through the network.

The workstations, hubs, and repeaters together form a LAN segment. A LAN segment is also known as a collision domain since collisions remain within the segment.

The area within which broadcasts and multicasts are confined is called a broadcast domain or LAN.

Thus a LAN can consist of one or more LAN segments. Defining broadcast and collision domains in a LAN depends on how the workstations, hubs, switches, and routers are physically connected together. This means that everyone on a LAN must be located in the same area (see Figure1).

Figure 1: Physical view of a LAN.

VLAN's allow a network manager to logically segment a LAN into different broadcast domains (see Figure2). Since this is a logical segmentation and not a physical one, workstations do not have to be physically located together. Users on different floors of the same building, or even in different buildings can now belong to the same LAN

Physical View

Logical View

Figure 2: Physical and logical view of a VLAN.

VLAN's also allow broadcast domains to be defined without using routers. Bridging software is used instead to define which workstations are to be included in the broadcast domain. Routers would only have to be used to communicate between two VLAN.

The acronym VLAN expands to Virtual Local Area Network. A VLAN is a logical local area network (or LAN) that extends beyond a single traditional LAN to a group of LAN segments, given specific configurations. Because a VLAN is a logical entity, its creation and configuration is done completely in software

Why use VLAN's?

VLAN's offer a number of advantages over traditional LAN's. They are:

1) Performance

In networks where traffic consists of a high percentage of broadcasts and multicasts, VLAN's can reduce the need to send such traffic to unnecessary destinations. For example, in a broadcast domain consisting of 10 users, if the broadcast traffic is intended only for 5 of the users, then placing those 5 users on a separate VLAN can reduce traffic

Compared to switches, routers require more processing of incoming traffic. As the volume of traffic passing through the routers increases, so does the latency in the routers, which results in

reduced performance. The use of VLAN's reduces the number of routers needed, since VLAN's create broadcast domains using switches instead of routers.

2) Formation of Virtual Workgroups

Nowadays, it is common to find cross-functional product development teams with members from different departments such as marketing, sales, accounting, and research. These workgroups are usually formed for a short period of time. During this period, communication between members of the workgroup will be high. To contain broadcasts and multicasts within the workgroup, a VLAN can be set up for them. With VLAN's it is easier to place members of a workgroup together. Without VLAN's, the only way this would be possible is to physically move all the members of the workgroup closer together.

However, virtual workgroups do not come without problems. Consider the situation where one user of the workgroup is on the fourth floor of a building, and the other workgroup members are on the second floor. Resources such as a printer would be located on the second floor, which would be inconvenient for the lone fourth floor user.

Another problem with setting up virtual workgroups is the implementation of centralized server farms, which are essentially collections of servers and major resources for operating a network at a central location. The advantages here are numerous, since it is more efficient and cost-effective to provide better security, uninterrupted power supply, consolidated backup, and a proper operating environment in a single area than if the major resources were scattered in a building. Centralized server farms can cause problems when setting up virtual workgroups if servers cannot be placed on more than one VLAN. In such a case, the server would be placed on a single VLAN and all other VLAN's trying to access the server would have to go through a router; this can reduce performance

3) Simplified Administration

Seventy percent of network costs are a result of adds, moves, and changes of users in the network Every time a user is moved in a LAN, rescaling, new station addressing, and reconfiguration of hubs and routers becomes necessary. Some of these tasks can be simplified with the use of VLAN's. If a user is moved within a VLAN, reconfiguration of routers is unnecessary. In addition, depending on the type of VLAN, other administrative work can be reduced or eliminated.

Despite this saving, VLAN's add a layer of administrative complexity, since it now becomes necessary to manage virtual workgroups

4) Reduced Cost

VLAN's can be used to create broadcast domains which eliminate the need for expensive routers.

5) Security

Periodically, sensitive data may be broadcast on a network. In such cases, placing only those users who can have access to that data on a VLAN can reduce the chances of an outsider gaining access to the data. VLAN's can also be used to control broadcast domains, set up firewalls, restrict access, and inform the network manager of an intrusion.

How VLAN's work

When a LAN bridge receives data from a workstation, it tags the data with a VLAN identifier indicating the VLAN from which the data came. This is called explicit tagging. It is also possible to determine to which VLAN the data received belongs using implicit tagging. In implicit tagging the data is not tagged, but the VLAN from which the data came is determined based on other information like the port on which the data arrived.

Tagging can be based on the port from which it came, the source Media Access Control (MAC) field, the source network address, or some other field or combination of fields.

VLAN's are classified based on the method used. To be able to do the tagging of data using any of the methods, the bridge would have to keep an updated database containing a mapping between VLAN's and whichever field is used for tagging. For example, if tagging is by port, the database should indicate which ports belong to which VLAN. This database is called a filtering database. Bridges would have to be able to maintain this database and also to make sure that all the bridges on the LAN have the same information in each of their databases. The bridge determines where the data is to go next based on normal LAN operations. Once the bridge determines where the data is to go, it now needs to determine whether the VLAN identifier should be added to the data and sent. If the data is to go to a device that knows about VLAN implementation (VLAN-aware), the VLAN identifier is added to the data. If it is to go to a device that has no knowledge of VLAN implementation (VLAN-unaware), the bridge sends the data without the VLAN identifier.

In order to understand how VLAN's work, we need to look at the types of VLAN's, the types of connections between devices on VLAN's, the filtering database which is used to send traffic to the correct VLAN, and tagging, a process used to identify the VLAN originating the data.

VLAN Standard: IEEE 802.1Q Draft Standard

There has been a recent move towards building a set of standards for VLAN products. The Institute of Electrical and Electronic Engineers (IEEE) is currently working on a draft standard 802.1Q for VLAN's. Up to this point, products have been proprietary, implying that anyone wanting to install VLAN's would have to purchase all products from the same vendor. Once the standards have been written and vendors create products based on these standards, users will no longer be confined to purchasing products from a single vendor.

Types of VLAN's

VLAN membership can be classified by port, MAC address, and protocol type.

1) Layer 1 VLAN: Membership by Port

Membership in a VLAN can be defined based on the ports that belong to the VLAN. For example, in a bridge with four ports, ports 1, 2, and 4 belong to VLAN 1 and port 3 belongs to VLAN 2 (see Figure3).

Port VLAN

1 1

2 1

3 2

4 1

Figure3: Assignment of ports to different VLAN's.

The main disadvantage of this method is that it does not allow for user mobility. If a user moves to a different location away from the assigned bridge, the network manager must reconfigure the VLAN.

2) Layer 2 VLAN: Membership by MAC Address

Here, membership in a VLAN is based on the MAC address of the workstation. The switch tracks the MAC addresses which belong to each VLAN (see Figure4). Since MAC addresses form a part of the workstation's network interface card, when a workstation is moved, no reconfiguration is needed to allow the workstation to remain in the same VLAN. This is unlike Layer 1 VLAN's where membership tables must be reconfigured.

MAC Address VLAN

1212354145121 1

2389234873743 2

3045834758445 2

5483573475843 1

Figure4: Assignment of MAC addresses to different VLAN's.

The main problem with this method is that VLAN membership must be assigned initially. In networks with thousands of users, this is no easy task. Also, in environments where notebook PC's are used, the MAC address is associated with the docking station and not with the notebook PC. Consequently, when a notebook PC is moved to a different docking station, its VLAN membership must be reconfigured.

3) Layer 2 VLAN: Membership by Protocol Type

VLAN membership for Layer 2 VLAN's can also be based on the protocol type field found in the Layer 2 header (see Figure5).

Protocol VLAN

IP 1

IPX 2

Figure5: Assignment of protocols to different VLAN's.

4) Layer 3 VLAN: Membership by IP Subnet Address

Membership is based on the Layer 3 header. The network IP subnet address can be used to classify VLAN membership (see Figure 6).

IP Subnet VLAN

23.2.24 1

26.21.35 2

Figure6: Assignment of IP subnet addresses to different VLAN's.

Although VLAN membership is based on Layer 3 information, this has nothing to do with network routing and should not be confused with router functions. In this method, IP addresses are used only as a mapping to determine membership in VLAN's. No other processing of IP addresses is done.

In Layer 3 VLAN's, users can move their workstations without reconfiguring their network addresses. The only problem is that it generally takes longer to forward packets using Layer 3 information than using MAC addresses.

5) Higher Layer VLAN's

It is also possible to define VLAN membership based on applications or service, or any combination thereof. For example, file transfer protocol (FTP) applications can be executed on one VLAN and telnet applications on another VLAN.

The 802.1Q draft standard defines Layer 1 and Layer 2 VLAN's only. Protocol type based VLAN's and higher layer VLAN's have been allowed for, but are not defined in this standard. As a result, these VLAN's will remain proprietary.

Types of Connections

Devices on a VLAN can be connected in three ways based on whether the connected devices are VLAN-aware or VLAN-unaware. Recall that a VLAN-aware device is one which understands VLAN memberships (i.e. which users belong to a VLAN) and VLAN formats.

1) Trunk Link

All the devices connected to a trunk link, including workstations, must be VLAN-aware. All frames on a trunk link must have a special header attached. These special frames are called tagged frames (see Figure7).

Figure7: Trunk link between two VLAN-aware bridges.

2) Access Link

An access link connects a VLAN-unaware device to the port of a VLAN-aware bridge. All frames on access links must be implicitly tagged (untagged) (see Figure8). The VLAN-unaware device can be a LAN segment with VLAN-unaware workstations or it can be a number of LAN segments containing VLAN-unaware devices (legacy LAN).

Figure 8: Access link between a VLAN-aware bridge and a VLAN-unaware device.

3) Hybrid Link

This is a combination of the previous two links. This is a link where both VLAN-aware and VLAN-unaware devices are attached (see Figure9). A hybrid link can have both tagged and untagged frames, but allthe frames for a specific VLAN must be either tagged or untagged.

Figure9: Hybrid link containing both VLAN-aware and VLAN-unaware devices.

It must also be noted that the network can have a combination of all three types of links.

Frame Processing

A bridge on receiving data determines to which VLAN the data belongs either by implicit or explicit tagging. In explicit tagging a tag header is added to the data. The bridge also keeps track of VLAN members in a filtering database which it uses to determine where the data is to be sent. Following is an explanation of the contents of the filtering database and the format and purpose of the tag header [802.1Q].

1) Filtering Database

Membership information for a VLAN is stored in a filtering database. The filtering database consists of the following types of entries:

i) Static Entries

Static information is added, modified, and deleted by management only. Entries are not automatically removed after some time (ageing), but must be explicitly removed by management. There are two types of static entries:

a) Static Filtering Entries: which specify for every port whether frames to be sent to a specific MAC address or group address and on a specific VLAN should be forwarded or discarded, or should follow the dynamic entry, and

b) Static Registration Entries: which specify whether frames to be sent to a specific VLAN are to be tagged or untagged and which ports are registered for that VLAN.

ii) Dynamic Entries

Dynamic entries are learned by the bridge and cannot be created or updated by management. The learning process observes the port from which a frame, with a given source address and VLAN ID (VID), is received, and updates the filtering database. The entry is updated only if all the following three conditions are satisfied:

a) this port allows learning,

b) the source address is a workstation address and not a group address, and

c) there is space available in the database.

Entries are removed from the database by the ageing out process where, after a certain amount of time specified by management (10 sec --- 1000000 sec), entries allow automatic reconfiguration of the filtering database if the topology of the network changes. There are three types of dynamic entries:

a) Dynamic Filtering Entries: which specify whether frames to be sent to a specific MAC address and on a certain VLAN should be forwarded or discarded.

b) Group Registration Entries: which indicate for each port whether frames to be sent to a group MAC address and on a certain VLAN should be filtered or discarded. These entries are added and deleted using Group Multicast Registration Protocol (GMRP). This allows multicasts to be sent on a single VLAN without affecting other VLAN's.

c) Dynamic Registration Entries: which specify which ports are registered for a specific VLAN. Entries are added and deleted using GARP VLAN Registration Protocol (GVRP), where GARP is the Generic Attribute Registration Protocol.

GVRP is used not only to update dynamic registration entries, but also to communicate the information to other VLAN-aware bridges.

In order for VLAN's to forward information to the correct destination, all the bridges in the VLAN should contain the same information in their respective filtering databases. GVRP allows both VLAN-aware workstations and bridges to issue and revoke VLAN memberships. VLAN-aware bridges register and propagate VLAN membership to all ports that are a part of the active topology of the VLAN. The active topology of a network is determined when the bridges are turned on or when a change in the state of the current topology is perceived.

The active topology is determined using a spanning tree algorithm which prevents the formation of loops in the network by disabling ports. Once an active topology for the network (which may contain several VLAN's) is obtained, the bridges determine an active topology for each VLAN. This may result in a different topology for each VLAN or a common one for several VLAN's. In either case, the VLAN topology will be a subset of the active topology of the network (see Figure 10).

Figure10: Active topology of network and VLAN A using spanning tree algorithm.

2) Tagging

When frames are sent across the network, there needs to be a way of indicating to which VLAN the frame belongs, so that the bridge will forward the frames only to those ports that belong to that VLAN, instead of

to all output ports as would normally have been done. This information is added to the frame in the form of a tag header. In addition, the tag header:

i) allows user priority information to be specified,

ii) allows source routing control information to be specified, and

iii) indicates the format of MAC addresses.

Frames in which a tag header has been added are called tagged frames. Tagged frames convey the VLAN information across the network.

The tagged frames that are sent across hybrid and trunk links contain a tag header. There are two formats of the tag header:

i) Ethernet Frame Tag Header: The ethernet frame tag header (see Figure11) consists of a tag protocol identifier (TPID) and tag control information (TCI).

Figure11: Ethernet frame tag header.

ii) Token Ring and Fiber Distributed Data Interface (FDDI) tag header: The tag headers for both token ring and FDDI networks consist of a SNAP-encoded TPID and TCI.

Figure12: Token ring and FDDI tag header.

TPID is the tag protocol identifier which indicates that a tag header is following and TCI (see Figure 13) contains the user priority, canonical format indicator (CFI), and the VLAN ID.

Figure13: Tag control information (TCI).

User priority is a 3 bit field which allows priority information to be encoded in the frame. Eight levels of priority are allowed, where zero is the lowest priority and seven is the highest priority. How this field is used is described in the supplement 802.1p.

The CFI bit is used to indicate that all MAC addresses present in the MAC data field are in canonical format. This field is interpreted differently depending on whether it is an ethernet-encoded tag header or a SNAP-encoded tag header. In SNAP-encoded TPID the field indicates the presence or absence of the canonical format of addresses. In ethernet-encoded TPID, it indicates the presence of the Source-Routing Information (RIF) field after the length field. The RIF field indicates routing on ethernet frames.

The VID field is used to uniquely identify the VLAN to which the frame belongs. There can be a maximum of (2 12 - 1) VLAN's. Zero is used to indicate no VLAN ID, but that user priority information is present. This allows priority to be encoded in non-priority LAN's.

VLAN modes

There are three different modes in which a VLAN can be configured. These modes are covered below:

VLAN Switching Mode - The VLAN forms a switching bridge in which frames are forwarded unmodified.

VLAN Translation Mode - VLAN translation mode is used when the frame tagging method is changed in the network path, or if the frame traverses from a VLAN group to a legacy or native interface which is not configured in a VLAN. When the packet is to pass into a native interface, the VLAN tag is removed so that the packet can properly enter the native interface.

VLAN Routing Mode - When a packet is routed from one VLAN to a different VLAN, you use VLAN routing mode. The packet is modified, usually by a router, which places its own MAC address as the source, and then changes the VLAN ID of the packet.

VLAN configurations

VLAN ID - The VLAN ID is a unique value you assign to each VLAN on a single device. With a Cisco routing or switching device running IOS, your range is from 1-4096. When you define a VLAN you usually use the syntax "vlan x" where x is the number you would like to assign to the VLAN ID. VLAN 1 is reserved as an administrative VLAN. If VLAN technologies are enabled, all ports are a member of VLAN 1 by default.

VLAN Name - The VLAN name is an text based name you use to identify your VLAN, perhaps to help technical staff in understanding its function. The string you use can be between 1 and 32 characters in length.

Private VLAN - You also define if the VLAN is to be a private vlan in the VLAN definition, and what other VLAN might be associated with it in the definition section. When you configure a Cisco VLAN as a private-vlan, this means that ports that are members of the VLAN cannot communicate directly with each other by default. Normally all ports which are members of a VLAN can communicate directly with each other just as they would be able to would they have been a member of a standard network segment. Private vlans are created to enhance the security on a network where hosts coexisting on the network cannot or should not trust each other. This is a common practice to use on web farms or in other high risk environments where communication between hosts on the same subnet are not necessary. Check your Cisco documentation if you have questions about how to configure and deploy private VLANs.

VLAN modes - in Cisco IOS, there are only two modes an interface can operate in, "mode access" and "mode trunk". Access mode is for end devices or devices that will not require multiple VLANs. Trunk mode is used for passing multiple VLANs to other network devices, or for end devices that need to have membership to multiple VLANs at once. If you are wondering what mode to use, the mode is probably "mode access".

VLAN Definition

To define a VLAN on a cisco device, you need a VLAN ID, a VLAN name, ports you would like to participate in the VLAN, and the type of membership the port will have with the VLAN.

Step 1 configure terminal Step 2 vlan vlan-id Step 3 name vlan-nameStep 4- If you want your new VLAN to be a private-vlan, you now enter "private-vlan primary" and "private-vlan association Y" where Y is the secondary VLAN you want to associate with the primary vlan. If you would like the private VLAN to be community based, you enter "private-vlan community" instead. Step 5 endStep 6 show vlan {name vlan-name | id vlan-id}

You have now created a vlan by assigning it an ID, and giving it a name. At this point, the VLAN has no special configuration to handle IP traffic, nor are there any ports that are members of the VLAN. The next section describes how you complete your vlan configuration.

VLAN Configuration Step 1 - Enter "Interface VlanX" where X is the VLAN ID you used in the VLAN definition

above. Step 2 - This step is optional. Enter "description " where VLAN description details what the

VLAN is going to be used for. You can just simply re-use the VLAN name you used above if you like.

Step 3 - Enter "ip address <address> <netmask>" where <address> is the address you want to assign this device in the VLAN, and <netmask> is the network mask for the subnet you have assigned the VLAN.

Step 4 - The step is optional. Create and apply an access list to the VLAN for inbound and outbound access controls. For a standard access list enter "access-group XXX in" and "access-group YYY out" where XXX and YYY corresponds to access-lists you have previously configured. Remember that the terms are taken in respect to the specific subnet or interface, so "in" means from the VLAN INTO the router, and "out" means from the router OUT to the VLAN.

Step 5 - This step is optional. Enter the private VLAN mapping you would like to use if the port is part of a private VLAN. This should be the same secondary VLAN you associated with the primary VLAN in VLAN definition above. Enter "private-vlan mapping XX" where XX is the VLAN ID of the secondary VLAN you would like to associate with this VLAN.

Step 6 - This step is optional. Configure HSRP and any other basic interface configurations you would normally use for your Cisco device.

Step 7 - Exit configuration mode by entering "end". Step 8 - Save your configuration to memory by entering "wr mem" and to the network if you have

need using "wr net". You may have to supply additional information to write configurations to the network depending on your device configuration.

Now you have your vlan defined and configured, but no physical ports are a member of the VLAN, so the VLAN still isn't of much use. Next port membership in the VLAN is described. IOS devices describe interfaces based on a technology and a port number, as with "FastEthernet3/1" or "GigabitEthernet8/16". Once you have determined which physical ports you want to be members of the VLAN you can use the following steps to configure it. NOTE: These steps have already assumed that you have logged into the router, gotten into enable mode, and entered configuration mode.

For access ports Step 1 - Enter "Interface <interface name>" where <interface name> is the name Cisco has

assigned the interface you would like to associate with the VLAN. Step 2 - This step is optional. Enter "description <interface description>" where <interface

description> is text describing the system connected to the interface in question. It is usually helpful to provide DNS hostname, IP Address, which port on the remote system is connected, and its function.

Step 3 - This step depends on your equipment and IOS version, and requirements. Enter "switchport" if you need the interface to act as a switch port. Some hardware does not support

switchport mode, and can only be used as a router port. Check your documentation if you don't know the difference between a router port and a switch port.

Step 4 - Only use this step if you used step 3 above. Enter "switchport access vlan X" where X is the VLAN ID of the VLAN you want the port to be a member of.

Step 5 - Only use this step if you used step 3 above. Enter "switchport mode access" to tell the port that you want it to be used as an access port.

Step 6 - Exit configuration mode by entering "end". Step 7 - Save your configuration to memory by entering "wr mem" and to the network if you have

need using "wr net". You may have to supply additional information to write configurations to the network depending on your device configuration.

For trunk ports

Step 1 - Enter "Interface <interface name>" where <interface name> is the name Cisco has assigned the interface you would like to associate with the VLAN.

Step 2 - This step is optional. Enter "description <interface description>" where <interface description> is text describing the system connected to the interface in question. It is usually helpful to provide DNS hostname, IP Address, which port on the remote system is connected, and its function.

Step 3 - This step depends on your equipment and IOS version, and requirements. Enter "switchport" if you need the interface to act as a switch port. Some hardware does not support switchport mode, and can only be used as a router port. Check your documentation if you don't know the difference between a router port and a switch port.

Step 4 - Only use this step if you used step 3 above. Enter "switchport trunk encapsulation dot1q". This tells the VLAN to use dot1q encapsulation for the VLAN, which is the industry standard encapsulation for trunking. There are other encapsulation options, but your equipment may not operate with non Cisco equipment if you use them.

Step 5 - Only use this step if you used step 3 above. Enter "switchport trunk allowed vlan XX, YY, ZZ" where XX, YY, and ZZ are VLANs you want the trunk to include. You can define one or more VLANs to be allowed in the trunk.

Step 6 - Only use this step if you used step 3 above. Enter "switchport mode trunk" to tell the port to operate as a VLAN trunk, and not as an access port.

Step 7 - Exit configuration mode by entering "end". Step 8 - Save your configuration to memory by entering "wr mem" and to the network if you have

need using "wr net". You may have to supply additional information to write configurations to the network depending on your device configuration.

For private VLAN ports Step 1 - Enter "Interface <interface name>" where <interface name> is the name Cisco has

assigned the interface you would like to associate with the VLAN. Step 2 - This step is optional. Enter "description <interface description>" where <interface

description> is text describing the system connected to the interface in question. It is usually helpful to provide DNS hostname, IP Address, which port on the remote system is connected, and its function.

Step 3 - This step depends on your equipment and IOS version, and requirements. Enter "switchport" if you need the interface to act as a switch port. Some hardware does not support switchport mode, and can only be used as a router port. Check your documentation if you don't know the difference between a router port and a switch port.

Step 4 - Enter "switchport private-vlan host association XX YY" where XX is the primary VLAN you want to assign, YY is the secondary VLAN you want to associate with it.

Step 5 - Enter "switchport mode private-vlan host" to force the port to operate as a private-vlan in host mode.

Step 6 - Exit configuration mode by entering "end".

Step 7 - Save your configuration to memory by entering "wr mem" and to the network if you have need using "wr net". You may have to supply additional information to write configurations to the network depending on your device configuration.

You should now have your VLAN properly implemented on a Cisco IOS device

Lecture notes # 16

Reading: - C6.1-6.6, C14.14.9, B164-200Contents: - Routing ConceptsStatic and Dynamic RoutingRouting Protocols: RIP, OSPF, and BGP

What is routing?Routing is used for taking a packet from one device and sending it through the network to another device on a different network. For this we use router.

Routers route traffic to all the networks in your internet work. To be able to route packets, a router must know, at a minimum, the following:

Destination address Neighbor routers from which it can learn about remote networks Possible routes to all remote networks The best route to each remote network How to maintain and verify routing information

Routing table:-The routing information a router learns from its routing source is placed in routing table.

At a minimum, each route entry in the database must contain two items:

Destination address This is the address of the network the router can reach. As this chapter explains, the router might have more than one route to the same address, or a group of subnets of the same or of varying lengths, grouped under the same major IP network address.

Pointer to the destination This pointer either will indicate that the destination network is directly connected to the router or it will indicate the address of another router on a directly connected link or the local interface to that link. That router, which will be one router hop closer to the destination, is a next-hop router.

The router will match the most specific address. The address may be one of the following:

Host address (a host route) Subnet

Group of subnets (a summary route)

Major network number

Group of major network numbers (a supernet)

Default address

Routing technique:-a) Next hop routing:-

In this the routing table will contains only the information that will leads to next hops. Its does not contain information about complete routing as shown in fig

b) Network specific routing:- In this routing table will contain only one entry which will define the address of network itself. It does not contain the entry of every host connected to same physical network as shown below

c) Host specific routing:- In this routing table will contain the destination host address in given routing table. This type of routing is used for specific purposes such as checking the route or providing security

measures. As shown above fig.

How routing table are used?Routers use the information in routing table to forwards packets as follows:-

1. When router receives a packet on interface it examines the destination address field.2. The router checks it routing table to see if it knows how to forward the packet towards the

destination:- If the destination network is not contained in routing table the router drops the packets If the destination network is contained in routing table the router checks the entry to see which

most desirable path for the packet to take is.

3. When it has determined the preferred path to the destination the router checks the routing table entry to see which of its interface leads to the next hop in that path. The next hop might be another intermediate router as the destination network itself.

4. The routers queues the packet at the appropriate interfaces & the packet are sent on its ways to the next hop in the path to the destination.

Different type of routing a) Static routingb) Default routingc) Dynamic routing

Static routing:-Static routing is the process of an administrator manually adding routes in each router’s routing table. In static routing algorithms, routes change very slowly over time, often as a result of human intervention (e.g., a human manually editing a router's forwarding table).

Static routing has the following benefits: No overhead on the router CPU No bandwidth usage between routers Security (because the administrator only allows routing to certain networks)

Static routing has the following disadvantages: The administrator must really understand the internet work and how each router is connected to

configure the routes correctly. If one network is added to the internet work, the administrator must add a route to it on all routers. It’s not feasible in large networks because it would be a full-time job.

The command used to add a static route to a routing table is

ip route [destination_network] [mask] [next_hop_address or exit interface] [administrative_distance][permanent]

The following list describes each command in the string:a) Ip route The command used to create the static route.b) Destination network The network you are placing in the routing table.c) Mask Indicates the subnet mask being used on the network.d) Next hop address The address of the next hop router that will receive the packet and forward

it to the remote network. This is a router interface that is on a directly connected network. You must be able to ping the router interface before you add the route.

e) Exit interface Used in place of the next hop address if desired. Must be on a point-to-point link, such as a WAN. This command does not work on a LAN; for example, Ethernet.

f) Administrative distance By default, static routes have an administrative distance of 1. You can change the default value by adding an administrative weight at the end of the command.

g) Permanent If the interface is shut down or the router cannot communicate to the next hop router, the route is automatically discarded from the routing table. Choosing the permanent option keeps the entry in the routing table no matter what happens.

Administrative DistancesWhen configuring routing protocols, you need to be aware of administrative distances (ADs). These are used to rate the trustworthiness of routing information received on a router from a neighbor router. An administrative distance is an integer from 0 to 255, where 0 is the most trusted and 255 means no traffic will be passed via this route.

Lab 5.1: Creating Static RoutesIn this first lab, you will create a static route in all four routers so that the routers see all networks. Verify with the Ping program when complete.1. The 2621 router is connected to network 172.16.10.0/24. It does not know about networks 172.16.20.0/24, 172.16.30.0/24, 172.16.40.0/24, and 172.16.50.0/24. Create static routes so that the 2621 router can see all networks, as shown here.2621#config t2621(config)#ip route 172.16.20.0 255.255.255.0 172.16.10.12621(config)#ip route 172.16.30.0 255.255.255.0 172.16.10.12621(config)#ip route 172.16.40.0 255.255.255.0 172.16.10.12621(config)#ip route 172.16.50.0 255.255.255.0 172.16.10.12. Save the current configuration for the 2621 router by going to the enabled mode, typing copy run start, and pressing Enter.3. On Router A, create a static route to see networks 172.16.10.0/24, 172.16.30.0/24, 172.16.40.0/24, and 172.16.50.0/24, as shown here.RouterA#config tRouterA(config)#ip route 172.16.30.0 255.255.255.0 172.16.20.2RouterA(config)#ip route 172.16.40.0 255.255.255.0 172.16.20.2RouterA(config)#ip route 172.16.50.0 255.255.255.0 172.16.20.2These commands told Router A to get to network 172.16.30.0/24 and use either IP address 172.16.20.2, which is the closet neighbor interface connected to network 172.16.30.0/24, or Router B. This is the same interface you will use to get to networks 172.16.40.0/24 and 172.16.50.0/24.4. Save the current configuration for Router A by going to the enabled mode, typing copy run start, and pressing Enter.5. On Router B, create a static route to see networks 172.16.10.0/24 and 172.16.50.0/24, which are not directly connected. Create static routes so that Router B can see all networks, as shown here.RouterB#config tRouterB(config)#ip route 172.16.10.0 255.255.255.0 172.16.20.1RouterB(config)#ip route 172.16.50.0 255.255.255.0 172.16.40.2

The first command told Router B that to get to network 172.16.10.0/24, it needs to use 172.16.20.1. The next command told Router B to get to network 172.16.50.0/24 through 172.16.40.2. Save the current configuration for Router B by going to the enable mode, typing copy run start, and pressing Enter.6. Router C is connected to networks 172.16.50.0/24 and 172.16.40.0/ 24. It does not know about networks 172.16.30.0/24, 172.16.20.0/ 24, and 172.16.10.0/24. Create static routes so that Router C can see all networks, as shown here.RouterC#config tRouterC(config)#ip route 172.16.30.0 255.255.255.0 172.16.40.1RouterC(config)#ip route 172.16.20.0 255.255.255.0 172.16.40.1RouterC(config)#ip route 172.16.10.0 255.255.255.0 172.16.40.1Save the current configuration for Router C by going to the enable mode, typing copy run start, and pressing Enter. Now ping from each router to your hosts and from each router to each router. If it is set up correctly, it will work.

Default RoutingDefault routing is used to send packets with a remote destination network not in the routing table to the next hop router. You can only use default routing on stub networks, which means that they have only one exit port out of the network.

To configure a default route, you use wildcards in the network address and mask locations of a static route.

Dynamic RoutingDynamic routing is the process of using protocols to find and update routing tables on routers. Dynamic routing algorithms change the routing paths as the network traffic loads (and the resulting delays experienced by traffic) or topology change.A dynamic algorithm can be run either periodically or in direct response to topology or link cost changes. While dynamic algorithms are more responsive to network changes, they are also more susceptible to problems such as routing loops and oscillation in routes, issues.

Advantage:-This is easier than static or default routingDisadvantage:-Expense of router CPU processes Bandwidth on the network links.

Lecture notes # 17

Reading: - C13.1-13.4Contents: - Routing Protocols: RIP, OSPF, and BGP

Routing protocol:-A routing protocol defines the set of rules used by a router when it communicates between neighbor routers.At the heart of any routing protocol is the algorithm (the "routing algorithm") that determines the path for a packet. The purpose of a routing algorithm is simple: given a set of routers, with links connecting the routers, a routing algorithm finds a "good" path from source to destination. Typically, a "good" path is one which has "least cost,"

The graph abstraction used to formulate routing algorithms is shown in Figure 4.2-1.Here, nodes in the graph represent routers - the points at which packet routing decisions are made - and the lines ("edges" in graph theory terminology) connecting these nodes represent the physical links between these routers. A link also has a value representing the "cost" of sending a packet across the link. The cost may reflect the level of congestion on that link (e.g., the current average delay for a packet across that link) or the physical distance traversed by that link (e.g., a transoceanic link might have a higher cost than a terrestrial link). For our current purposes, we will simply take the link costs as a given and won't worry about how they are determined.

Given the graph abstraction, the problem of finding the least cost path from a source to a destination requires identifying a series of links such that:

1. the first link in the path is connected to the source2. the last link in the path is connected to the destination3. for all i, the i and i-1st link in the path are connected to the same node4. for the least cost path, the sum of the cost of the links on the path is the minimum over all

possible paths between the source and destination. Note that if all link costs are the same, the least cost path is also the shortest path (i.e., the path crossing the smallest number of links between the source and the destination).

In Figure 4.2-1, for example, the least cost path between nodes A (source) and C (destination) is along the path ADEC.

Classification of Routing Algorithms

A global routing algorithm computes the least cost path between a source and destination using complete, global knowledge about the network. That is, the algorithm takes the connectivity between all nodes and all links costs as inputs. This then requires that the algorithm somehow obtain this information before actually performing the calculation. The calculation itself can be run at one site (a centralized global routing

algorithm) or replicated at multiple sites. The key distinguishing feature here, however, is that a global algorithm has complete information about connectivity and link costs. In practice, algorithms with global state information are often referred to as link state algorithms, since the algorithm must be aware of the state (cost) of each link in the network. In a decentralized routing algorithm, the calculation of the least cost path is carried out in an iterative, distributed manner. No node has complete information about the costs of all network links. Instead, each node begins with only knowledge of the costs of its own directly attached links and then through an iterative process of calculation and exchange of information with its neighboring nodes (i. e., nodes which are at the "other end" of links to which it itself is attached) gradually calculates the least cost path to a destination, or set of destinations. a decentralized routing algorithm known as a distance vector algorithm. It is called a distance vector algorithm because a node never actually knows a complete path from source to destination. Instead, it only knows the direction (which neighbor) to which it should forward a packet in order to reach a given destination along the least cost path, and the cost of that path from itself to the destination.There are three classes of routing protocols:

a) Distance vector b) Link state c) Path vector routingd) Hybrid

Above 3 routing protocols are used for updating routing table.

Routing updates are necessary to keeps the entire router in an internet work informed of changes in network topology.

Depending on routing protocol a routing update might contain all or just small part of routing table

A second broad way to classify routing algorithms is according to whether they are static or dynamic.

Routing Protocol Basics

All dynamic routing protocols are built around an algorithm. Generally, an algorithm is a step-by-step procedure for solving a problem. A routing algorithm must, at a minimum, specify the following:

A procedure for passing reachability information about networks to other routers A procedure for receiving reachability information from other routers

A procedure for determining optimal routes based on the reachability information it has and for recording this information in a route table

A procedure for reacting to, compensating for, and advertising topology changes in a network

A few issues common to any routing protocol are path determination, metrics, convergence, and load balancing.

Path Determination

All subnets within a network must be connected to a router, and wherever a router has an interface on a network, that interface must have an address on the network. This address is the originating point for reachability information.

Metrics

When there are multiple routes to the same destination, a router must have a mechanism for calculating the best path. A metric is a variable assigned to routes as a means of ranking them from best to worst or from most preferred to least preferred.

Hop Count

A hop-count metric simply counts router hops.

Bandwidth

A bandwidth metric would choose a higher-bandwidth path over a lower-bandwidth link. However, bandwidth by itself still might not be a good metric. What if one or both of the T1 links are heavily loaded with other traffic and the 56K link is lightly loaded? Or what if the higher-bandwidth link also has a higher delay?

Load

This metric reflects the amount of traffic utilizing the links along the path. The best path is the one with the lowest load.

Unlike hop count and bandwidth, the load on a route changes, and, therefore, the metric will change. Care must be taken here. If the metric changes too frequently, route flapping the frequent change of preferred routes might occur. Route flaps can have adverse effects on the router's CPU, the bandwidth of the data links, and the overall stability of the network.

Delay

Delay is a measure of the time a packet takes to traverse a route. A routing protocol using delay as a metric would choose the path with the least delay as the best path. There might be many ways to measure delay. Delay might take into account not only the delay of the links along the route, but also such factors as router latency and queuing delay. On the other hand, the delay of a route might not be measured at all; it might be a sum of static quantities defined for each interface along the path. Each individual delay quantity would be an estimate based on the type of link to which the interface is connected.

Reliability

Reliability measures the likelihood that the link will fail in some way and can be either variable or fixed. Examples of variable-reliability metrics are the number of times a link has failed, or the number of errors it has received within a certain time period. Fixed-reliability metrics are based on known qualities of a link as determined by the network administrator. The path with highest reliability would be selected as best.

Cost

This metric is configured by a network administrator to reflect more- or less-preferred routes. Cost might be defined by any policy or link characteristic or might reflect the arbitrary judgment of the network administrator. Therefore, "cost" is a term of convenience describing a dimensionless metric.

Convergence

A dynamic routing protocol must include a set of procedures for a router to inform other routers about its directly connected networks, to receive and process the same information from other routers, and to pass along the information it receives from other routers. Further, a routing protocol must define a metric by which best paths might be determined.

The continuous circling of traffic between two or more destinations is referred to as a routing loop.

The process of bringing all route tables to a state of consistency is called convergence. The time it takes to share information across a network and for all routers to calculate best paths is the convergence time.

Load Balancing

That load balancing is the practice of distributing traffic among multiple paths to the same destination, so as to use bandwidth efficiently. As an example of the usefulness of load balancing. For e.g. A network can be a reachable from two paths. If a device on 192.168.2.0 sends a stream of packets to a device on 192.168.6.0, Router A might send them all via Router B or Router C. In both cases, the network is one hop away. However, sending all packets on a single route probably is not the most efficient use of available bandwidth. Instead, load balancing should be implemented to alternate traffic between the two paths., load balancing can be equal cost or unequal cost, and per packet or per destination.

Distance Vector Routing Protocols

It is also known as Bellman-Ford routing algorithm.

The distance-vector routing algorithm passes complete routing tables to neighbor routers. The neighbor routers then combine the received routing table with their own routing tables to complete the internet work map. This is called routing by rumor, because a router receiving an update from a neighbor router believes the information about remote networks without actually finding out for itself. It is possible to have a network that has multiple links to the same remote network. If that is the case, the administrative distance is first checked. If the administrative distance is the same, it will have to use other metrics to determine the best path to use to that remote network.

The distance vector (DV) algorithm is iterative, asynchronous, and distributed. It is distributed in that each node receives some information from one or more of its directly attached neighbors, performs a calculation, and may then distribute the results of its calculation back to its neighbors. It is iterative in that this process continues on until no more information is exchanged between neighbors. (Interestingly, we will see that the algorithm is self terminating -- there is no "signal" that the computation should stop; it just stops).The algorithm is asynchronous in that it does not require all of the nodes to operate in lock step with each other.

The principal data structure in the DV algorithm is the distance table maintained at each node. Each node's distance table has a row for each destination in the network and a column for each of its directly attached neighbors. Consider a node X that is interested in routing to destination Y via its directly attached neighbor Z. Node X's distance table entry, Dx(Y,Z) is the sum of the cost of the direct one hop link between X and Z, c(X,Z), plus neighbor Z's currently known minimum cost path from itself (Z) to Y. That is:

The minw term in above equation is taken over all of Z's directly attached neighbors (including X, as we shall soon see).Above Equation suggests the form of the neighbor-to-neighbor communication that will take place in the DV algorithm -- each node must know the cost of each of its neighbors minimum cost path to each destination Thus, whenever a node computes a new minimum cost to some destination, it must inform its neighbors of this new minimum cost.

Before presenting the DV algorithm, let's consider an example that will help clarify the meaning of entries in the distance table.

Consider the network topology and the distance table shown for node E in Figure 4.2-3. This is the distance table in node E once the Dv algorithm has converged. Let's first look at the row for destination A.Clearly the cost to get to A from E via the direct connection to A has a cost of 1. Hence

Let's now consider the value of - the cost to get from E to A, given that the first step along the path is D. In this case, the distance table entry is the cost to get from E to D (a cost of 2) plus whatever the minimum cost it is to get from D to A . Note that the minimum cost from D to A is 3 -- a path that passes right back through E! Nonetheless, we record the fact that the minimum cost from E to A given that the first step is via D has a cost of 5. Similarly, we find that the distance table entry via

neighbor B is

A circled entry in the distance table gives the cost of the least cost path to the corresponding destination (row). The column with the circled entry identifies the next node along the least cost path to the destination. Thus, a node's routing table (which indicates which outgoing link should be used to forward packets to a given destination) is easily constructed from the node's distance table. In discussing the distance table entries for node E above, we informally took a global view, knowing the costs of all links in the network. The distance vector algorithm we will now present is decentralized and does not use such global information. Indeed, the only information a node will have are the costs of the links to its directly attached neighbors, and information it receives from these directly attached neighbors. It is used in many routing algorithms in practice, including: Internet BGP, ISO IDRP, Novell IPX, and the original ARPAnet.

The key steps are lines 15 and 21, where a node updates its distance table entries in response to either a change of cost of an attached link or the receipt of an update message from a neighbor. The other key step is line 24, where a node sends an update to its neighbors if its minimum cost path to a destination has changedFigure 4.2-4 illustrates the operation of the DV algorithm for the simple three node network shown at the top of the figure. The operation of the algorithm is illustrated in a synchronous manner, where all nodes simultaneously receive messages from their neighbors, compute new distance table entries, and inform their neighbors of any changes in their new least path costs. After studying this example, you should convince yourself that the algorithm operates correctly in an asynchronous manner as well, with node computations and update generation/ reception occurring at any times.The circled distance table entries in Figure 4.2-4 show the current least path cost to a destination. An entry circled in red indicates that a new minimum cost has been computed (in either line 4 of the DV algorithm (initialization) or line 21). In such cases an update message will be sent (line 24 of the DV algorithm) to the node's neighbors as represented by the red arrows between columns in Figure 4.2-4.

The leftmost column in Figure 4.2-4 shows the distance table entries for nodes X, Y, and Z after the initialization step.Let us now consider how node X computes the distance table shown in the middle column of Figure 4.2-4 after receiving updates from nodes Y and Z. As a result of receiving the updates from Y and Z, X computes in line 21 of the DV algorithm:

It is important to note that the only reason that X knows about the terms

is because nodes Z and Y have sent those values to X (and are received by X in line 10 of the DV algorithm). As an exercise, verify the distance tables computed by Y

and Z in the middle column of Figure 4.2-4. The value means that X's minimum cost to Z has changed from 7 to 3. Hence, X sends updates to Y and Z informing them of this new least cost to Z. Note that X need not update Y and Z about its cost to Y since this has not changed. Note also that Y's recomputation of its distance table in the middle column of Figure 4.2-4 does result in new distance entries, but does not result in a change of Y's least cost path to nodes X and Z. Hence Y does not send updates to X and Z. The process of receiving updated costs from neighbors, recomputation of distance table entries, and updating neighbors of changed costs of the least cost path to a destination continues until no update messages are sent. At this point, since no update messages are sent, no further distance table calculations will occur and the algorithm enters a quiescent state, i.e., all nodes are performing the wait in line 9 of the DV algorithm. The algorithm would remain in the quiescent state until a link cost changes, as discussed below.

Main disadvantage of distance vector routing are:-a) pinhole congestionb) routing loops

To solve these problems we have following techniques:a) maximum hop countsb) splits horizonc) route poisoningd) holds downe) Triggered Updatesf) Asynchronous Updates

Pinhole congestion:-

Since network 172.16.30.0 is a T1 link with a bandwidth of 1.544Mbps, and network 172.16.20.0 is a 56K link, you would want the router to choose the T1 over the 56K link. However, since hop count is the only metric used with RIP routing, they would both be seen as equal-cost links. This is called pinhole congestion.It is important to understand what happens when a distance-vector routing protocol does when it starts up. In Figure 5.6, the four routers start off with only their directly connected networks in the routing table. After a distance-vector routing protocol is started on each router, the routing tables are updated with all route information gathered from neighbor routers.

As shown in Figure 5.6, each router has only the directly connected networks in each routing table. Each router sends its complete routing table out to each active interface on the router. The routing table of each router includes the network number, exit interface, and hop count to the network. In Figure 5.7, the routing tables are complete because they include information about all the networks in the internet work. They are considered converged. When the routers are converging, no data is passed. That’s why fast convergence time is a plus. One of the problems with RIP, in fact, is its slow convergence time.

The routing tables in each router keep information regarding the network number, the interface to which the router will send packets out to get to the remote network, and the hop count or metric to the remote network.

Routing Loops

Distance-vector routing protocols keep track of any changes to the internet work by broadcasting periodic routing updates to all active interfaces. This broadcast includes the complete routing table. This works fine, although it takes up CPU process and link bandwidth. However, if a network outage happens, problems can occur. The slow convergence of distance-vector routing protocols can cause inconsistent routing tables and routing loops. Routing loops can occur because every router is not updated close to the same time. Let’s say that the interface to Network 5 in Figure 5.8 fails. All routers know about Network 5 from Router E. Router A, in its tables, has a path to Network 5 through Routers B, C, and E. When Network 5 fails, Router E tells RouterC. This causes Router C to stop routing to Network 5 through Router E. But Routers A, B, and D don’t know about Network 5 yet, so they keep sending out update information. Router C will eventually send out its update and cause B to stop routing to Network 5, but Routers A and D are still not updated. To them, it appears that Network 5 is still available through Router B with a metric of three.

Router A sends out its regular 30-second “Hello, I’m still here—these are the links I know about” message, which includes reachability for Network 5. Routers B and D then receive the wonderful news that Network 5 can be reached from Router A, so they send out the information that Network 5 is available. Any packet destined for Network 5 will go to Router A, to Router B, and then back to Router A. This is a routing loop—how do you stop it?

Maximum Hop Count

The routing loop problem just described is called counting to infinity, and it’s caused by gossip and wrong information being communicated and propagated throughout the internet work. Without some form of intervention, the hop count increases indefinitely each time a packet passes through a router.One way of solving this problem is to define a maximum hop count. Distance vector (RIP) permits a hop count of up to 15, so anything that requires 16 hops is deemed unreachable. In other words, after a loop of 15 hops, Network 5 will be considered down. This means that counting to infinity will keep packets from going around the loop forever. Though this is a workable solution, it won’t remove the routing loop itself.

Packets will still go into the loop, but instead of traveling on unchecked, they’ll whirl around for 16 bounces and die

Split Horizon

Another solution to the routing loop problem is called split horizon. This reduces incorrect routing information and routing overhead in a distance vector network by enforcing the rule that information cannot be sent back in the direction from which it was received. It would have prevented Router A from sending the updated information it received from Router B back to Router B.

Route Poisoning

Another way to avoid problems caused by inconsistent updates is route poisoning. For example, when Network 5 goes down, Router E initiates route poisoning by entering a table entry for Network 5 as 16, or unreachable (sometimes referred to as infinite). By this poisoning of the route to Network 5, Router C is not susceptible to incorrect updates about the route to Network 5. When Router C receives a router poisoning from Router E, it sends an update, called a poison reverse, back to Router E. This makes sure all routes on the segment have received the poisoned route information. Route poisoning, used with hold downs (discussed next), will speed up convergence time because neighboring routers don’t have to wait 30 seconds (an eternity in computer land) before advertising the poisoned route.

Hold downs

And then there are hold downs. These prevent regular update messages from reinstating a route that has gone down. Hold downs also help prevent routes from changing too rapidly by allowing time for either the downed route to come back or the network to stabilize somewhat before changing to the next best route. These also tell routers to restrict, for a specific time period, any changes that might affect recently removed routes. This prevents inoperative routers from being prematurely restored to other routers’ tables.When a router receives an update from a neighbor indicating that a previously accessible network is not working and is inaccessible, the hold down timer will start. If a new update arrives from a neighbor with a better metric than the original network entry, the hold down is removed and data is passed. However, if an update is received from a neighbor router before the hold down timer expires and it has a lower metric than the previous route, the update is ignored and the hold down timer keeps ticking. This allows more time for the network to converge.Hold downs use triggered updates, which reset the hold down timer, to alert the neighbor routers of a change in the network. Unlike update messages from neighbor routers, triggered updates create a new routing table that is sent immediately to neighbor routers because a change was detected in the internet work.There are three instances when triggered updates will reset the hold down timer:1. The hold down timer expires.2. The router receives a processing task proportional to the number of links in the internet work.3. Another update is received indicating the network status has changed.

Triggered Updates

Triggered updates, also known as flash updates, are very simple: If a metric changes for better or for worse, a router will immediately send out an update without waiting for its update timer to expire. Reconvergence will occur far more quickly than if every router had to wait for regularly scheduled updates, and the problem of counting to infinity is greatly reduced, although not completely eliminated. Regular updates might still occur along with triggered updates. Thus a router might receive bad information about a route from a not-yet-reconverged router after having received correct information from a triggered update. Such a situation shows that confusion and routing errors might still occur while a network is reconverging, but triggered updates will help to iron things out more quickly.

A further refinement is to include in the update only the networks that actually triggered it, rather than the entire route table. This technique reduces the processing time and the impact on network bandwidth.

Asynchronous Updates

a group of routers connected to an Ethernet backbone. The routers should not broadcast their updates at the same time; if they do, the update packets will collide. Yet this situation is exactly what can happen when several routers share a broadcast network. System delays related to the processing of updates in the routers tend to cause the update timers to become synchronized. As a few routers become synchronized, collisions will begin to occur, further contributing to system delays and eventually all routers sharing the broadcast network might become synchronized.

Asynchronous updates might be maintained by one of two methods:

Each router's update timer is independent of the routing process and is, therefore, not affected by processing loads on the router.

A small random time, or timing jitter, is added to each update period as an offset

Examples of distance-vector routing protocols are RIP and IGRP.

Link State Routing Protocol

These algorithms use the principle of a link state to determine network topology. A link state is the description of an interface on a router (for example, IP address, subnet mask, type of network) and its relationship to neighboring routers. The collection of these link states forms a link state database. The process used by link state algorithms to determine network topology is straightforward:1. Each router identifies all other routing devices on the directly connected networks.2. Each router advertises a list of all directly connected network links and the associated cost of each link. This is performed through the exchange of link state advertisements (LSAs) with other routers in the network.3. Using these advertisements, each router creates a database detailing the current network topology. The topology database in each router is identical.4. Each router uses the information in the topology database to compute the most desirable routes to each destination network. This information is used to update the IP routing table.

The link state algorithm we present below is known as Dijkstra's algorithm, named after its inventor .It computes the least cost path from one node (the source, which we will refer to as A) to all other nodes in the network. Dijkstra's algorithm is iterative and has the property that after the kth iteration of the algorithm, the least cost paths are known to k destination nodes, and among the least cost paths to all destination nodes, these k path will have the k smallest costs. Let us define the following notation:

c(i,j): link cost from node i to node j. If nodes i and j are not directly connected, then c(i,j) = infty. We will assume for simplicity that c(i,j) equals c(j,i).

D(v): the cost of path from the source node to destination v that has currently (as of this iteration of the algorithm) the least cost.

p(v): previous node (neighbor of v) along current least cost path from source to v N: set of nodes whose shortest path from the source is definitively known

The link state algorithm consists of an initialization step followed by a loop. The number of times the loop is executed is equal to the number of nodes in the network. Upon termination, the algorithm will have calculated the shortest paths from the source node to every other node in the network.

As an example, let us consider the network in Figure 4.2-1 and compute the shortest path from A to all possible destinations. A tabular summary of the algorithm's computation is shown in Table 4.2-1, where each line in the table gives the values of the algorithms variables at the end of the iteration. Let us consider the few first steps in detail:

In the initialization step, the currently known least path costs from A to its directly attached neighbors, B, C and D are initialized to 2, 5 and 1 respectively. Note in particular that the cost to C is set to 5 (even though we will soon see that a lesser cost path does indeed exists) since this is cost of the direct (one hop) link from A to C. The costs to E and F are set to infinity since they are not directly connected to A.

In the first iteration, we look among those nodes not yet added to the set N and find that node with the least cost as of the end of the previous iteration. That node is D, with a cost of 1, and thus

D is added to the set N. Line 12 of the LS algorithm is then performed to update D(v) for all nodes v, yielding the results shown in the second line (step 1) in Table 4.2-1. The cost of the path to B is unchanged. The cost of the path to C (which was 5 at the end of the initialization) through node D is found to have a cost of 4. Hence this lower cost path is selected and C's predecessor along the shortest path from A is set to D. Similarly, the cost to E (through D) is computed to be 2, and the table is updated accordingly.

In the second iteration, nodes B and E are found to have the shortest path costs (2), and we break the tie arbitrarily and add E to the set N so that N now contains A, D, and E. The cost to the remaining nodes not yet in N, i.e., nodes B, C and F, are updated via line 12 of the LS algorithm , yielding the results shown in the third row in the above table.

and so on ...

When the LS algorithm terminates, we have for each node, its predecessor along the least cost path from the source node. For each predecessor, we also have its predecessor and so in this manner we can construct the entire path from the source to all destinations.

In the first iteration, we need to search through all n nodes to determine the node, w, not in N that has the minimum cost. In the second iteration, we need to check n-1 nodes to determine the minimum cost; in the third iteration n-2 nodes and so on. Overall, the total number of nodes we need to search through over all the iterations is n*(n+1)/2, and thus we say that the above implementation of the link state algorithm has worst case complexity of order n squared: O(n2). (A more sophisticated implementation of this algorithm, using a data structure known as a heap, can find the minimum in line 9 in logarithmic rather than linear time, thus reducing the complexity).

An example of an IP routing protocol that is completely link state is OSPF

Path vector routing

The path vector routing algorithm is somewhat similar to the distance vector algorithm in the sense that each border router advertises the destinations it can reach to its neighboring router. However, instead of advertising networks in terms of a destination and the distance to that destination, networks are advertised as destination addresses and path descriptions to reach those destinations.

A route is defined as a pairing between a destination and the attributes of the path to that destination, thus the name, path vector routing, where the routers receive a vector that contains paths to a set of destinations.

The path, expressed in terms of the domains (or confederations) traversed so far, is carried in a special path attribute that records the sequence of routing domains through which the reachability information has passed. The path represented by the smallest number of domains becomes the preferred path to reach the destination.

The main advantage of a path vector protocol is its flexibility. There are several other advantages regarding using a path vector protocol:

The computational complexity is smaller than that of the link state protocol. The path vector computation consists of evaluating a newly arrived route and comparing it with the existing one, while conventional link state computation requires execution of an SPF algorithm.

Path vector routing does not require all routing domains to have homogeneous policies for route selection; route selection policies used by one routing domain are not necessarily known to other routing domains. The support for heterogeneous route selection policies has serious implications for the computational complexity. The path vector protocol allows each domain to make its route selection autonomously, based only on local policies. However, path vector routing can accommodate heterogeneous route selection with little additional cost.

Only the domains whose routes are affected by the changes have to recomputed. Suppression of routing loops is implemented through the path attribute, in contrast to link state

and distance vector, which use a globally-defined monotonically thereby increasing metric for

route selection. Therefore, different confederation definitions are accommodated because looping is avoided by the use of full path information.

Route computation precedes routing information dissemination. Therefore, only routing information associated with the routes selected by a domain is distributed to adjacent domains.

Path vector routing has the ability to selectively hide information. However, there are disadvantages to this approach, including:

Topology changes only result in the recomputation of routes affected by these changes, which is more efficient than complete recomputation. However, because of the inclusion of full path information with each distance vector, the effect of a topology change can propagate farther than in traditional distance vector algorithms.

Unless the network topology is fully meshed or is able to appear so, routing loops can become an issue.

BGP is a popular example of a path vector routing protocol.

Hybrid routing

The last category of routing protocols is hybrid protocols. These protocols attempt to combine the positive attributes of both distance vector and link state protocols. Like distance vector, hybrid protocols use metrics to assign a preference to a route. However, the metrics are more accurate than conventional distance vector protocols. Like link state algorithms, routing updates in hybrid protocols are event driven rather than periodic. Networks using hybrid protocols tend to converge more quickly than networks using distance vector protocols.Finally, these protocols potentially reduce the costs of link state updates and distance vector advertisements.Although open hybrid protocols exist, this category is almost exclusively associated with the proprietary EIGRP algorithm.

A Comparison of Link State and Distance Vector Routing Algorithms

Message Complexity. We have seen that LS requires each node to know the cost of each link in the network. This requires O(nE) messages to be sent, where n is the number of nodes in the network and E is the number of links. Also, whenever a link cost changes, the new link cost must be sent to all nodes. The DV algorithm requires message exchanges between directly connected neighbors at each iteration. We have seen that the time needed for the algorithm to converge can depend on many factors. When link costs change, the DV algorithm will propagate the results of the changed link cost only if the new link cost results in a changed least cost path for one of the nodes attached to that link.

Speed of Convergence. We have seen that our implementation of the LS is an O(n2) algorithm requiring O(nE) messages, and potentially suffer from oscillations. The DV algorithm can converge slowly (depending on the relative path costs,) and can have routing loops while the algorithm is converging. DV also suffers from the count to infinity problem.

Robustness. What can happen is a router fails, misbehaves, or is sabotaged? Under LS, a router could broadcast an incorrect cost for one of its attached links (but no others). A node could also corrupt or drop any LS broadcast packets it receives as part of link state broadcast. But an LS node is only computing its own routing tables; other nodes are performing the similar calculations for themselves. This means route calculations are somewhat separated under LS, providing a degree of robustness. Under DV, a node can advertise incorrect least path costs to any/all destinations. More generally, we note that at each iteration, a node's calculation in DV is passed on to its neighbor and then indirectly to its neighbor's neighbor on the next iteration. In this sense, an incorrect node calculation can be diffused through the entire network under DV.

In the end, neither algorithm is a "winner" over the other; as both algorithms are used in the Internet.

What is difference between?

a) centralized vs. distributed routingb) interdomain vs. intradomain routingc) host based vs. router based routingd) unicast vs. multicast routing

a) Centralized vs. distributed routingCentralized:-In a centralized routing environment a single router collects & distributes topology information for all part of internet work.Advantage:

Its relieves other routers in the inter network of responsibility of route collection.Disadvantage:-

Network links from the central router to other router carry a disproportionate amount of traffic. If central routers fails other routers do not receives routing updates so to remove this problem we

use backup central routers.Distributed routing:-In a distributed routing environment all routers in the internet work share the responsibility for collecting, distributing & using internet work topology information.Advantage:

Self sufficiency of individual router makes the routing environment more tolerant of routing failures.

Also traffic is evenly distributed among networks links.Disadvantage:-

It is that there are significantly more relationships established between routers & all routers are burdened with route calculation & other processing tasks.

b) Interdomain vs. Intradomain routingInterdomain: - (it’s also called exterior routing)This type of routing occurs between multiple autonomous systems. E.g. BGP

Intradomain routing: - (it is also called interior routing)In this routing occurs only within autonomous system e.g. IGRP

Autonomous system:-It is a group of networks & routers under the authority of a single administration is called autonomous system.

c) Host based vs. router based routingRouter based routing:-

Routers are responsible for determining the route to a destination through the network Routers make routing decisions based on their own calculations The router will consider the entire best path based on various measures. Path selected is not optimal No discovery traffic is generated Decision making process is very rapid

Host based routing: - (same as host specific routing) Source end is responsible for determining the route to a destination through internet network. Here router acts as store & forward devices simply sending packets to next devices in the path. The source end node discovers all possible route to a destination before the packet is sent into the

internet work It then choice best optimal path It often require substantial discovery traffic It takes significant amount of time.

d) Unicast vs. multicast routingUnicast routing:-

In unicast routing there is one source and one destination.

The address for both source & destination is unicast address assign to host. In Uincast routing when a router receives a packet it forward the packet through only one of its

ports

Multicast routing:-In multicast routing there is one source & group of destination.Source address is unicast address & destination address is group of address (class D)Group of address: - its define the members of group

DVMRP- DISTANCE VECTOR MULTICAST ROUTING PROTOCOLMOSPF- MULTICAST OPEN SHORTEST PATH FIRST PROTOCOLPIM – PROTOCOL INDEPENDENT MULTICASTPIM-DM- PROTOCOL INDEPENDENT MULTICAST DENSE MODEPIM-SM- PROTOCOL INDEPENDENT MULTICAST SPARSE MODECBT- CORE BASED TREE

Routing Protocols: RIP, OSPF, and BGP

RIP

UNICAST ROUTING PROTOCOL

INTERIOR ROUTING EXTERIOR ROUTING

IGRP RIP OSPFBGP

MULTICAST ROUTING PROTOCOL

SOURCE BASED TREE GROUP SHARED TREE

DVMRP MOSPF

PIMP

PIM-DM

PIM-SM

CBT

RIP is a distance vector protocol using hop count as a routing metric to measure the distance between the source and a destination network. Each link is assigned a hop-count value (which is 1 typically). RIP routers maintain only the best route (the route with the lowest hop count value) to a destination in their routing tables. Each RIP router sends routing-update messages at regular intervals and when the network topology changes. When a router receives a routing update message that indicates a route change, it updates its routing table and immediately sends routing-update messages to inform its neighbors about the change.

RIP uses a number of timers in routing, 1. The route-update timer. Clocks the interval between periodic routing updates, and is generally set to 30 seconds plus a small, randomly generated number of seconds to avoid collisions.2. The route-invalid timer. A route becomes invalid when it is not updated over a period defined by this timer. The route is marked as inaccessible and advertised as unreachable .However, the router still forwards packets to this route until the flush interval (see below) expires. The default value is 180 seconds.3. The route-hold-down timer. The interval during which routing information regarding better paths is suppressed. The interval should be at least three times the value of the update timer. A route enters into a hold down state when an update packet is received indicating the route is unreachable. The default value is 180 seconds.4. The route-flush timer. Amount of time that must pass before the route is removed from the routing table. The interval should be longer than the larger of the invalid and hold-down values. The default value is 240 seconds.

RIP packet typesThe RIP protocol specifies two packet types. These packets can be sent by any device running the RIP protocol:Request packets: A request packet queries neighboring RIP devices to obtain their distance vector table. The request indicates if the neighbor should return either a specific subset or the entire contents of the table.Response packets: A response packet is sent by a device to advertise the information maintained in its local distance vector table. The table is sent during the following situations:

The table is automatically sent every 30 seconds. The table is sent as a response to a request packet generated by another RIP node. If triggered updates are supported, the table is sent when there is a change to the local distance

vector table. When a response packet is received by a device, the information contained in the update is

compared against the local distance vector table. If the update contains a lower cost route to a destination, the table is updated to reflect the new path.

RIP modes of operationRIP hosts have two modes of operation:

Active mode: Devices operating in active mode advertise their distance vector table and also receive routing updates from neighboring RIP hosts. Routing devices are typically configured to operate in active mode.

Passive (or silent) mode: Devices operating in this mode simply receive routing updates from neighboring RIP devices. They do not advertise their distance vector table. End stations are typically configured to operate in passive mode.

RIP messages format RIP messages are encapsulated in UDP datagrams, using the well-known port number 520. Figure 4.4 shows the format of a RIP message, and Fig. 4.5 shows the format of a RIP-2 message. The fields of a RIP message are listed here.

Command: Indicates whether the packet is a request (1) or a response (2). Version Number: Specifies the RIP version used (1 or 2). Address-Family Identifier: Specifies the address family used. RIP can be used to carry routing

information for several different protocol families. For IP, this field is 2. Address: Specifies the IP address for the entry. Metric: Indicates how many hops have been traversed from the source to the destination.

The RIP-2 message takes advantage of the unused fields in RIP, and provides additional information such as subnet support and a simple authentication scheme. These fields are listed here.

Routing Domain: The identifier of the routing daemon that sends this message (e.g., the process ID of the routing daemon).

Route Tag: Used to support EGPs, carrying the AS number. Subnet Mask: The subnet mask associated with the IP address advertised. Next-hop IP Address: Where IP datagrams to the advertised IP address should be forwarded to.

RIP is widely used because of its simplicity and low routing overhead. However, it has the Count-to-Infinity problem which causes routing loops. To solve this problem, RIP uses a hop-count limit of 15.

NOTE: - RIP version 1 uses only classful routing, which means that all devices in the network must use the same subnet mask. This is because RIP version 1 does not send updates with subnet mask information in tow. RIP version 2 provides what is called prefix routing and does send subnet mask information with the route updates. This is called classless routing.

Configuring RIP

To configure RIP routing, just turn on the protocol with the router rip command and tell the RIP routing protocol which networks to advertise.

Lab 5.2: Dynamic Routing with RIPIn this lab, we will use the dynamic routing protocol RIP instead of static and default routing.1. Remove any static routes or default routes configured on your routers by using the no ip route command. For example:RouterA#config tRouterA(config)#no ip route 172.16.10.0 255.255.255.0 172.16.11.2RouterA(config)#no ip route 172.16.30.0 255.255.255.0 172.16.20.2RouterA(config)#no ip route 172.16.40.0 255.255.255.0 172.16.20.2RouterA(config)#no ip route 172.16.50.0 255.255.255.0 172.16.20.2RouterA(config)#no ip route 172.16.55.0 255.255.255.0 172.16.20.2Do the same thing for Routers B and C and the 2621 router. Type sh run and press Enter on each router to verify that all static and default routes are cleared.2. After your static and default routers are clear, go into configuration mode on Router A by typing config t.3. Tell your router to use RIP routing by typing router rip and pressingEnter, as shown here:config trouter rip4. Add the network number you want to advertise by typing network 172.16.0.0 and pressing Enter.5. Press Ctrl+Z to get out of configuration mode.6. Go to Routers B and C and the 2621 router and type the same commands, as shown here:Config tRouter ripNetwork 172.16.0.07. Verify that RIP is running at each router by typing the following commands at each router:show ip protocolshow ip routeshow running-config or show run8. Save your configurations by typing copy run start or copy runningconfig startup-config and pressing Enter at each router.9. Verify the network by pinging all remote networks and hosts.

OSPF

The Open Shortest Path First (OSPF) protocol is another example of an interior gateway protocol. It was developed as a non-proprietary routing alternative to address the limitations of RIP.The following features contribute to the continued acceptance of the OSPF standard:

Equal cost load balancing: The simultaneous use of multiple paths can provide more efficient utilization of network resources.

Logical partitioning of the network: This reduces the propagation of outage information during adverse conditions. It also provides the ability to aggregate routing announcements that limit the advertisement of unnecessary subnet information.

Support for authentication: OSPF supports the authentication of any node transmitting route advertisements. This prevents fraudulent sources from corrupting the routing tables.

Faster convergence time: OSPF provides instantaneous propagation of routing changes. This expedites the convergence time required to update network topologies.

Support for CIDR and VLSM: This allows the network administrator to efficiently allocate IP address resources.

OSPF is a link state protocol. As with other link state protocols, each OSPF router executes the SPF algorithm to process the information stored in the link state database. The algorithm produces a shortest-path tree detailing the preferred routes to each destination network.

OSPF terminology

OSPF uses specific terminology to describe the operation of the protocol.OSPF areasOSPF networks are divided into a collection of areas. An area consists of a logical grouping of networks and routers. The area can coincide with geographic or administrative boundaries. Each area is assigned a 32-bit area ID.Subdividing the network provides the following benefits:

Within an area, every router maintains an identical topology database describing the routing devices and links within the area. These routers have no knowledge of topologies outside the area. They are only aware of routes to these external destinations. This reduces the size of the topology database maintained by each router.

Areas limit the potentially explosive growth in the number of link state updates. Most LSAs are distributed only within an area.

Areas reduce the CPU processing required to maintain the topology database. The SPF algorithm is limited to managing changes within the area.

Backbone area and area 0All OSPF networks contain at least one area. This area is known as area 0 or the backbone areaIn networks containing multiple areas, the backbone physically connects to all other areas. OSPF expects all areas to announce routing information directly into the backbone. The backbone then announces this information into other areas.Figure 5-14 depicts a network with a backbone area and four additional areas

Intra-area, area border, and AS boundary routersThere are three classifications of routers in an OSPF network. Figure 5-14 illustrates the interaction of these devices.Intra-area routers :- This class of router is logically located entirely within an OSPF area. Intra-area routers maintain a topology database for their local area.Area border routers (ABR) :- This class of router is logically connected to two or more areas. One area must be the backbone area. An ABR is used to interconnect areas. They maintain a separate topology database for each attached area. ABRs also execute separate instances of the SPF algorithm for each area.AS boundary routers (ASBR) :- This class of router is located at the periphery of an OSPF internetwork. It functions as a gateway exchanging reachability between the OSPF network and other routing environments.

Each router is assigned a 32-bit router ID (RID). The RID uniquely identifies the device

Physical network types

OSPF categorizes network segments into three types. The frequency and types of communication occurring between OSPF devices connected to these networks is impacted by the network type:1. Point-to-point: Point-to-point networks directly link two routers.2. Multi-access: Multi-access networks support the attachment of more than two routers.They are further subdivided into two types:

Broadcast networks have the capability of simultaneously directing a packet to all attached routers. This capability uses an address that is recognized by all devices. Ethernet and token-ring LANs are examples of OSPF broadcast multi-access networks.

Non-broadcast networks do not have broadcasting capabilities. Each packet must be specifically addressed to every router in the network. X.25 and frame relay networks are examples of OSPF non-broadcast multi-access networks.

3. Point-to-multipoint: Point-to-multipoint networks are a special case of multi-access, non-broadcast networks. In a point-to-multipoint network, a device is not required to have a direct connection to every other device. This is known as a partially meshed environment.

Neighbor routers and adjacencies

Routers that share a common network segment establish a neighbor relationship on the segment. Routers must agree on the following information to become neighbors:

Area ID: The routers must belong to the same OSPF area. Authentication: If authentication is defined, the routers must specify the same password. Hello and dead intervals: The routers must specify the same timer intervals used in the Hello

protocol. Stub area flag: The routers must agree that the area is configured as a stub area.

After two routers have become neighbors, an adjacency relationship can be formed between the devices. Neighboring routers are considered adjacent when they have synchronized their topology databases. This occurs through the exchange of link state information.

Designated and backup designated router

The exchange of link state information between neighbors can create significant quantities of network traffic. To reduce the total bandwidth required to synchronize databases and advertise link state information, a router does not necessarily develop adjacencies with every neighboring device:

Multi-access networks: Adjacencies are formed between an individual router and the (backup) designated router.

Point-to-point networks: An adjacency is formed between both devices.

Each multi-access network elects a designated router (DR) and backup designated router (BDR). The DR performs two key functions on the network segment:

It forms adjacencies with all routers on the multi-access network. This causes the DR to become the focal point for forwarding LSAs.

It generates network link advertisements listing each router connected to the multi-access network

The BDR forms the same adjacencies as the designated router. It assumes DR functionality when the DR fails.Each router is assigned an 8-bit priority, indicating its ability to be selected as the DR or BDR. A router priority of zero indicates that the router is not eligible to be selected. The priority is configured on each interface in the router.

Figure 5-15 illustrates the relationship between neighbors. No adjacencies are formed between routers that are not selected to be the DR or BDR.

Link state database

The link state database is also called the topology database. It contains the set of link state advertisements describing the OSPF network and any external connections. Each router within the area maintains an identical copy of the link state database.

Link state advertisements and flooding

The contents of an LSA describe an individual network component (that is, router, segment, or external destination). LSAs are exchanged between adjacent OSPF routers. This is done to synchronize the link state database on each device.When a router generates or modifies an LSA, it must communicate this change throughout the network. The router starts this process by forwarding the LSA to each adjacent device. Upon receipt of the LSA, these neighbors store the information in their link state database and communicate the LSA to their neighbors. This store and forward activity continues until all devices receive the update. This process is called reliable flooding. Two steps are taken to ensure that this flooding effectively transmits changes without overloading the network with excessive quantities of LSA traffic:

Each router stores the LSA for a period of time before propagating the information to its neighbors. If, during that time, a new copy of the LSA arrives, the router replaces the stored version. However, if the new copy is outdated, it is discarded.

To ensure reliability, each link state advertisement must be acknowledged. Multiple acknowledgements can be grouped together into a single acknowledgement packet. If an acknowledgement is not received, the original link state update packet is retransmitted.

Link state advertisements contain five types of information. Together these advertisements provide the necessary information needed to describe the entire OSPF network and any external environments:Router LSAs: This type of advertisement describes the state of the router's interfaces (links) within the area. They are generated by every OSPF router. The advertisements are flooded throughout the area.Network LSAs: This type of advertisement lists the routers connected to a multi-access network. They are generated by the DR on a multi-access segment. The advertisements are flooded throughout the area.Summary LSAs (Type-3 and Type-4): This type of advertisement is generated by an ABR. There are two types of summary link advertisements:Type-3 summary LSAs describe routes to destinations in other areas within the OSPF network (inter-area destinations).Type-4 summary LSAs describe routes to ASBRs. Summary LSAs are used to exchange reachability information between areas. Normally, information is announced into the backbone area. The backbone then injects this information into other areas.

AS external LSAs: This type of advertisement describes routes to destinations external to the OSPF network. They are generated by an ASBR. The advertisements are flooded throughout all areas in the OSPF network.

OSPF packet typesOSPF packets are transmitted in IP datagrams. They are not encapsulated within TCP or UDP packets. The IP header uses protocol identifier 89. OSPF packets are sent with an IP ToS of 0 and an IP precedence of internetwork control. This is used to obtain preferential processing for the packets. Wherever possible, OSPF uses multicast facilities to communicate with neighboring devices. In broadcast and point-to-point environments, packets are sent to the reserved multicast address 224.0.0.5.In non-broadcast environments, packets are addressed to the neighbor’s specific IP address.All OSPF packets share the common header shown in Figure 5-17. The header provides general information including area identifier, RID, checksum, and authentication information.

The type field identifies the OSPF packet as one of five possible types:

Hello :- This packet type discovers and maintains neighbor relationships.Database description : This packet type describes the set of LSAs contained in the router's link state database.Link state request : This packet type requests a more current instance of an LSA from a neighbor.Link state update : This packet type provides a more current instance of an LSA to a neighbor.Link state acknowledgement : This packet type acknowledges receipt of a newly received LSA.

Neighbor communication

OSPF is responsible for determining the optimum set of paths through a network. To accomplish this, each router exchanges LSAs with other routers in the network. The OSPF protocol defines a number of activities to accomplish this information exchange:

Discovering neighbors Electing a designated router Establishing adjacencies and synchronizing databases

The five OSPF packet types are used to support these information exchanges.

Discovering neighbors: The OSPF Hello protocolThe Hello protocol discovers and maintains relationships with neighboring routers. Hello packets are periodically sent out to each router interface. The packet contains the RID of other routers whose hello packets have already been received over the interface.When a device sees its own RID in the hello packet generated by another router, these devices establish a neighbor relationship.The hello packet also contains the router priority, DR identifier, and BDR identifier. These parameters are used to elect the DR on multi-access networks.

Electing a designated routerAll multi-access networks must have a DR. A BDR can also be selected. The backup ensures there is no extended loss of routing capability if the DR fails. The DR and BDR are selected using information contained in hello packets. The device with the highest OSPF router priority on a segment becomes the DR for that segment. The same process is repeated to select the BDR. In case of a tie, the router with the highest RID is selected. A router declared the DR is ineligible to become the BDR. After elected, the DR and BDR proceed to establish adjacencies with all routers on the multi-access segment.

Establishing adjacencies and synchronizing databasesNeighboring routers are considered adjacent when they have synchronized their link state databases. A router does not develop an adjacency with every neighboring device. On multi-access networks, adjacencies are formed only with the DR and BDR. This is a two step process.Step 1: Database exchange processThe first phase of database synchronization is the database exchange process. This occurs immediately after two neighbors attempt to establish an adjacency. The process consists of an exchange of database description packets. The packets contain a list of the LSAs stored in the local database.During the database exchange process, the routers form a master/subordinate relationship. The master is the first to transmit. Each packet is identified by a sequence number. Using this sequence number, the subordinate acknowledges each database description packet from the master. The subordinate also includes its own set of link state headers in the acknowledgements.Step 2: Database loadingDuring the database exchange process, each router notes the link state headers for which the neighbor has a more current instance (all advertisements are time stamped). After the process is complete, each router requests the more current information from the neighbor. This request is made with a link state request packet.When a router receives a link state request, it must reply with a set of link state update packets providing the requested LSA. Each transmitted LSA is acknowledged by the receiver. This process is similar to the reliable flooding procedure used to transmit topology changes throughout the network.Every LSA contains an age field indicating the time in seconds since the origin of the advertisement. The age continues to increase after the LSA is installed in the topology database. It also increases during each hop of the flooding process.When the maximum age is reached, the LSA is no longer used to determining routing information and is discarded from the link state database. This age is also used to distinguish between two otherwise identical copies of an advertisement.

OSPF neighbor state machine

The OSPF specification defines a set of neighbor states and the events that can cause a neighbor to transition from one state to another. A state machine is used to describe these transitions:

Down: This is the initial state. It indicates that no recent information has been received from any device on the segment.

Attempt: This state is used on non-broadcast networks. It indicates that a neighbor appears to be inactive. Attempts continue to reestablish contact.

Init: Communication with the neighbor has started, but bidirectional communication has not been established. Specifically, a hello packet was received from the neighbor, but the local router was not listed in the neighbor's hello packet.

2-way: Bidirectional communication between the two routers has been established. Adjacencies can be formed. Neighbors are eligible to be elected as designated routers.

ExStart: The neighbors are starting to form an adjacency. Exchange: The two neighbors are exchanging their topology databases. Loading: The two neighbors are synchronizing their topology databases. Full: The two neighbors are fully adjacent and their databases are synchronized.

OSPF virtual links and transit areas

Virtual links are used when a network does not support the standard OSPF network topology. This topology defines a backbone area that directly connects to each additional OSPF area. The virtual link addresses two conditions:

It can logically connect the backbone area when it is not contiguous. It can connect an area to the backbone when a direct connection does not exist.

A virtual link is established between two ABRs sharing a common non-backbone area. The link is treated as a point-to-point link. The common area is known as a transit area. Figure 5-18 illustrates the interaction between virtual links and transit areas when used to connect an area to the backbone.

This diagram shows that area 1 does not have a direct connection to the backbone. Area 2 can be used as a transit area to provide this connection. A virtual link is established between the two ABRs located in area 2. Establishing this virtual link logically extends the backbone area to connect to area 1.A virtual link is used only to transmit routing information. It does not carry regular traffic between the remote area and the backbone. This traffic, in addition to the virtual link traffic, is routed using the standard intra-area routing within the transit area.

OSPF route redistributionRoute redistribution is the process of introducing external routes into an OSPF network. These routes can be either static routes or routes learned through another routing protocol. They are advertised into the OSPF network by an ASBR. These routes become OSPF external routes. The ASBR advertises these routes by flooding OSPF AS external LSAs throughout the entire OSPF network.The routes describe an end-to-end path consisting of two portions:

External portion: This is the portion of the path external to the OSPF network. When these routes are distributed into OSPF, the ASBR assigns an initial cost. This cost represents the external cost associated with traversing the external portion of the path.

Internal portion: This is the portion of the path internal to the OSPF network. Costs for this portion of the network are calculated using standard OSPF algorithms.

OSPF differentiates between two types of external routes. They differ in the way the cost of the route is calculated. The ASBR is configured to redistribute the route as:

External type 1: The total cost of the route is the sum of the external cost and any internal OSPF costs.

External type 2: The total cost of the route is always the external cost. This ignores any internal OSPF costs required to reach the ASBR.

Figure 5-19 illustrates an example of the types of OSPF external routes.

In this example, the ASBR is redistributing the 10.99.5.0/24 route into the OSPF network. This subnet is located within the RIP network. The route is announced into OSPF with an external cost of 50. This represents the cost for the portion of the path traversing the RIP network: If the ASBR redistributed the route as an E1 route, R1 will contain an external route to this subnet

with a cost of 60 (50 + 10). R2 will have an external route with a cost of 65 (50 + 15). If the ASBR redistributed the route as an E2 route, both R1 and R2 will contain an external route

to this subnet with a cost of 50. Any costs associated with traversing segments within the OSPF network are not included in the total cost to reach the destination.

OSPF stub areas

OSPF allows certain areas to be defined as a stub area. A stub area is created when the ABR connecting to a stub area excludes AS external LSAs from being flooded into the area. This is done to reduce the size of the link state database maintained within the stub area routers. Because there are no specific routes to external networks, routing to these destinations is based on a default route generated by the ABR. The link state databases maintained within the stub area contain only the default route and the routes from within the OSPF environment (for example, intra-area and inter-area routes).Because a stub area does not allow external LSAs, a stub area cannot contain an ASBR. No external routes can be generated from within the stub area.Stub areas can be deployed when there is a single exit point connecting the area to the backbone. An area with multiple exit points can also be a stub area. However, there is no guarantee that packets exiting the area will follow an optimal path. This is due to the fact that each ABR generates a default route. There is no ability to associate traffic with a specific default routes.All routers within the area must be configured as stub routers. This configuration is verified through the exchange of hello packets.

Not-so-stubby areas

An extension to the stub area concept is the not-so-stubby area (NSSA). An NSSA is similar to a stub area in that the ABR servicing the NSSA does not flood any external routes into the NSSA.The only routes flooded into the NSSA are the default route and any other routes from within the OSPF environment (for example, intra-area and inter-area).However, unlike a stub area, an ASBR can be located within an NSSA. This ASBR can generate external routes. Therefore, the link state databases maintained within the NSSA contain the default route, routes from within the OSPF environment (for example, intra-area and inter-area routes), and the external routes generated by the ASBR within the area.

The ABR servicing the NSSA floods the external routes from within the NSSA throughout the rest of the OSPF network.

OSPF route summarization

Route summarization is the process of consolidating multiple contiguous routing entries into a single advertisement. This reduces the size of the link state database and the IP routing table. In an OSPF network, summarization is performed at a border router. There are two types of summarization:

Inter-area route summarization: Inter-area summarization is performed by the ABR for an area. It is used to summarize route advertisements originating within the area. The summarized route is announcement into the backbone. The backbone receives the aggregated route and announces the summary into other areas.

External route summarization: This type of summarization applies specifically to external routes injected into OSPF. This is performed by the ASBR distributing the routes into the OSPF network.

Figure 5-20 illustrates an example of OSPF route summarization.

In this figure, the ASBR is advertising a single summary route for the 64 subnetworks located in the RIP environment. This single summary route is flooded throughout the entire OSPF network. In addition, the ABR is generating a single summary route for the 64 subnetworks located in area 1. This summary route is flooded through area 0 and area 2. Depending of the configuration of the ASBR, the inter-area summary route can also be redistributed into the RIP network.

A Basic OSPF Configuration

The three steps necessary to begin a basic OSPF process are

1. Determine the area to which each router interface will be attached.2. Enable OSPF with the command router ospf process-id.

3. Specify the interfaces on which to run OSPF, and their areas, with the network area command.

Example 8-19. Rubens's OSPF network area configuration.

router ospf 10 network 0.0.0.0 255.255.255.255 area 1

Example 8-20. Chardin's OSPF network area configuration.

router ospf 20 network 192.168.30.0 0.0.0.255 area 1 network 192.168.20.0 0.0.0.255 area 0

Example 8-21. Goya's OSPF network area configuration.

router ospf 30 network 192.168.20.0 0.0.0.3 area 0.0.0.0 network 192.168.10.0 0.0.0.31 area 192.168.10.0

Example 8-22. Matisse's OSPF network area configuration.

router ospf 40 network 192.168.10.2 0.0.0.0 area 192.168.10.0 network 192.168.10.33 0.0.0.0 area 192.168.10.0

Short note Operation of OSPF

At a very high level, the operation of OSPF is easily explained:

1. OSPF-speaking routers send Hello packets out all OSPF-enabled interfaces. If two routers sharing a common data link agree on certain parameters specified in their respective Hello packets, they will become neighbors.

2. Adjacencies, which can be thought of as virtual point-to-point links, are formed between some neighbors. OSPF defines several network types and several router types. The establishment of an adjacency is determined by the types of routers exchanging Hellos and the type of network over which the Hellos are exchanged.

3. Each router sends link-state advertisements (LSAs) over all adjacencies. The LSAs describe all of the router's links, or interfaces, the router's neighbors, and the state of the links. These links might be to stub networks (networks with no other router attached), to other OSPF routers, to networks

in other areas, or to external networks (networks learned from another routing process). Because of the varying types of link-state information, OSPF defines multiple LSA types.

4. Each router receiving an LSA from a neighbor records the LSA in its link-state database and sends a copy of the LSA to all of its other neighbors.

5. By flooding LSAs throughout an area, all routers will build identical link-state databases.

6. When the databases are complete, each router uses the SPF algorithm to calculate a loop-free graph describing the shortest (lowest cost) path to every known destination, with itself as the root. This graph is the SPF tree.

7. Each router builds its route table from its SPF tree

BGP:-

BGP performs interdomain routing in Transmission-Control Protocol/Internet Protocol (TCP/IP) networks. BGP is an exterior gateway protocol (EGP), which means that it performs routing between multiple autonomous systems or domains and exchanges routing and reachability information with other BGP systems.BGP was developed to replace its predecessor, the now obsolete Exterior Gateway Protocol (EGP), as the standard exterior gateway-routing protocol used in the global Internet. BGP solves serious problems with EGP and scales to Internet growth more efficiently.Figure 35-1 illustrates core routers using BGP to route traffic between autonomous systems.

BGP OperationBGP performs three types of routing: interautonomous system routing, intra-autonomous system routing, and pass-through autonomous system routing

Interautonomous system routing occurs between two or more BGP routers in different autonomous systems. Peer routers in these systems use BGP to maintain a consistent view of the internetwork topology. BGP neighbors communicating between autonomous systems must reside on the same physical network. The Internet serves as an example of an entity that uses this type of routing because it is comprised of autonomous systems or administrative domains. Many of these domains represent the various institutions, corporations, and entities that make up the Internet. BGP is frequently used to provide path determination to provide optimal routing within the Internet.

Intra-autonomous system routing occurs between two or more BGP routers located within the same autonomous system. Peer routers within the same autonomous system use BGP to maintain a consistent view of the system topology. BGP also is used to determine which router will serve as the connection point for specific external autonomous systems. Once again, the Internet provides an example of interautonomous system routing. An organization, such as a university, could make use of BGP to provide optimal routing within its own administrative domain or autonomous system. The BGP protocol can provide both inter- and intra-autonomous system routing services.

Pass-through autonomous system routing occurs between two or more BGP peer routers that exchange traffic across an autonomous system that does not run BGP. In a pass-through autonomous system environment, the BGP traffic did not originate within the autonomous system in question and is not destined for a node in the autonomous system. BGP must interact with whatever intra-autonomous system routing protocol is being used to successfully transport BGP traffic through that autonomous system. Figure 35-2 illustrates a pass-through autonomous system environment:

BGP RoutingAs with any routing protocol, BGP maintains routing tables, transmits routing updates, and bases routing decisions on routing metrics. The primary function of a BGP system is to exchange network-reachability information, including information about the list of autonomous system paths, with other BGP systems. This information can be used to construct a graph of autonomous system connectivity from which routing loops can be pruned and with which autonomous system-level policy decisions can be enforced.

Each BGP router maintains a routing table that lists all feasible paths to a particular network. The router does not refresh the routing table, however. Instead, routing information received from peer routers is retained until an incremental update is received.

BGP devices exchange routing information upon initial data exchange and after incremental updates. When a router first connects to the network, BGP routers exchange their entire BGP routing tables. Similarly,

when the routing table changes, routers send the portion of their routing table that has changed. BGP routers do not send regularly scheduled routing updates, and BGP routing updates advertise only the optimal path to a network.

BGP uses a single routing metric to determine the best path to a given network. This metric consists of an arbitrary unit number that specifies the degree of preference of a particular link. The BGP metric typically is assigned to each link by the network administrator. The value assigned to a link can be based on any number of criteria, including the number of autonomous systems through which the path passes, stability, speed, delay, or cost.

BGP Message TypesThe open message opens a BGP communications session between peers and is the first message sent by each side after a transport-protocol connection is established. Open messages are confirmed using a keep-alive message sent by the peer device and must be confirmed before updates, notifications, and keep-alives can be exchanged.An update message is used to provide routing updates to other BGP systems, allowing routers to construct a consistent view of the network topology. Updates are sent using the Transmission-Control Protocol (TCP) to ensure reliable delivery. Update messages can withdraw one or more unfeasible routes from the routing table and simultaneously can advertise a route while withdrawing others.

The notification message is sent when an error condition is detected. Notifications are used to close an active session and to inform any connected routers of why the session is being closed.

The keep-alive message notifies BGP peers that a device is active. Keep-alives are sent often enough to keep the sessions from expiring.

BGP Packet FormatsHeader FormatAll BGP message types use the basic packet header. Open, update, and notification messages have additional fields, but keep-alive messages use only the basic packet header. Figure 35-3 illustrates the fields used in the BGP header. The section that follows summarizes the function of each field.

BGP Packet-Header FieldsEach BGP packet contains a header whose primary purpose is to identify the function of the packet in question. The following descriptions summarize the function of each field in the BGP header illustrated in Figure 35-3.• Marker— Contains an authentication value that the message receiver can predict.• Length— Indicates the total length of the message in bytes.• Type—Type — Specifies the message type as one of the following:— Open— Update— Notification— Keep-alive• Data—Contains upper-layer information in this optional field.

Open Message FormatBGP open messages are comprised of a BGP header and additional fields. Figure 35-4 illustrates the additional fields used in BGP open messages.

BGP Open Message FieldsBGP packets in which the type field in the header identifies the packet to be a BGP open message packet include the following fields. These fields provide the exchange criteria for two BGP routers to establish a peer relationship.• Version—Provides the BGP version number so that the recipient can determine whether it is running the same version as the sender.• Autonomous System—Provides the autonomous system number of the sender.• Hold-Time—Indicates the maximum number of seconds that can elapse without receipt of a message before the transmitter is assumed to be nonfunctional.• BGP Identifier—Provides the BGP identifier of the sender (an IP address), which is determined at startup and is identical for all local interfaces and all BGP peers.• Optional Parameters Length—Indicates the length of the optional parameters field (if present).• Optional Parameters—Contains a list of optional parameters (if any). Only one optional parameter type is currently defined: authentication information. Authentication information consists of the following two fields:— Authentication code: Indicates the type of authentication being used.— Authentication data: Contains data used by the authentication mechanism (if used).Update Message FormatBGP update messages are comprised of a BGP header and additional fields. Figure 35-5 illustrates the additional fields used in BGP update messages.

BGP Update Message FieldsBGP packets in which the type field in the header identifies the packet to be a BGP update message packet include the following fields. Upon receiving an update message packet, routers will be able to add or delete specific entries from their routing tables to ensure accuracy. Update messages consist of the following packets:• Unfeasible Routes Length—Indicates the total length of the withdrawn routes field or that the field is not present.• Withdrawn Routes—Contains a list of IP address prefixes for routes being withdrawn from service.

• Total Path Attribute Length—Indicates the total length of the path attributes field or that the field is not present.• Path Attributes—Describes the characteristics of the advertised path. The following are possible attributes for a path:— Origin: Mandatory attribute that defines the origin of the path information— AS Path: Mandatory attribute composed of a sequence of autonomous system path segments— Next Hop: Mandatory attribute that defines the IP address of the border router that should be used as the next hop to destinations listed in the network layer reachability information field— Mult Exit Disc: Optional attribute used to discriminate between multiple exit points to a neighboring autonomous system— Local Pref: Discretionary attribute used to specify the degree of preference for an advertised route— Atomic Aggregate: Discretionary attribute used to disclose information about route selections— Aggregator: Optional attribute that contains information about aggregate routes• Network Layer Reachability Information—Contains a list of IP address prefixes for the advertised routesNotification Message FormatFigure 35-6 illustrates the additional fields used in BGP notification messages.

BGP Notification Message FieldsBGP packets in which the type field in the header identifies the packet to be a BGP notification message packet include the following fields. This packet is used to indicate some sort of error condition to the peers of the originating router.• Error Code—indicates the type of error that occurred. The following are the error types defined by the field:— Message Header Error: Indicates a problem with a message header, such as unacceptable message length, unacceptable marker field value, or unacceptable message type.— Open Message Error: Indicates a problem with an open message, such as unsupported version number, unacceptable autonomous system number or IP address, or unsupported authentication code.— Update Message Error: Indicates a problem with an update message, such as a malformed attribute list, attribute list error, or invalid next-hop attribute.— Hold Time Expired: Indicates that the hold-time has expired, after which time a BGP node will be considered nonfunctional.— Finite State Machine Error: Indicates an unexpected event.— Cease: Closes a BGP connection at the request of a BGP device in the absence of any fatal errors.• Error Subcode—Provides more specific information about the nature of the reported error.• Error Data—Contains data based on the error code and error subcode fields. This field is used to diagnose the reason for the notification message.

BGP concepts and terminologyBGP uses specific terminology to describe the operation of the protocol. Figure 5-21 illustrates this terminology.

BGP uses the following terms:_ BGP speaker: A router configured to support BGP._ BGP neighbors (peers): A pair of BGP speakers that exchange routing information. There are two types of BGP neighbors:– Internal (IBGP) neighbor: A pair of BGP speakers within the same AS.– External (EBGP) neighbor: A pair of BGP neighbors, each in a different AS. These neighbors typically share a directly connected network._ BGP session: A TCP session connecting two BGP neighbors. The session is used to exchange routing information. The neighbors monitor the state of the session by sending keepalive messages._ Traffic type: BGP defines two types of traffic:– Local: Traffic local to an AS either originates or terminates within the AS.Either the source or the destination IP address resides in the AS.– Transit: Any traffic that is not local traffic is transit traffic. One of the goals of BGP is to minimize the amount of transit traffic._ AS type: BGP defines three types of autonomous systems:– Stub: A stub AS has a single connection to one other AS. A stub AS carries only local traffic.– Multihomed: A multihomed AS has connections to two or more autonomous systems. However, a multihomed AS has been configured so that it does not forward transit traffic.– Transit: A transit AS has connections to two or more autonomous systems and carries both local and transit traffic. The AS can impose policy restrictions on the types of transit traffic that will be forwardedDepending on the configuration of the BGP devices within AS 2 in Figure 5-, this autonomous system can be either a multihomed AS or a transit AS._ AS number: A 16-bit number uniquely identifying an AS._ AS path: A list of AS numbers describing a route through the network. A BGP neighbor communicates paths to its peers._ Routing policy: A set of rules constraining the flow of data packets through the network. Routing policies are not defined in the BGP protocol. Rather, they are used to configure a BGP device. For example, a BGP device can be configured so that:– A multihomed AS can refuse to act as a transit AS. This is accomplished by advertising only those networks contained within the AS.– A multihomed AS can perform transit AS routing for a restricted set of adjacent autonomous systems. It does this by tailoring the routing advertisements sent to EBGP peers.– An AS can optimize traffic to use a specific AS path for certain categories of traffic.

_ Network layer reachability information (NLRI): NLRI is used by BGP to advertise routes. It consists of a set of networks represented by the tuple <length,prefix>. For example, the tuple <14,220.24.106.0> represents the CIDR route 220.24.106.0/14._ Routes and paths: A route associates a destination with a collection of attributes describing the path to the destination. The destination is specified in NRLI format. The path is reported as a collection of path attributes. This information is advertised in UPDATE messages

How BGP Selects Paths

A router running Cisco IOS Release 12.0 or later does not select or use an iBGP route unless both of the following are true:

• The router has a route available to the next-hop.

• If synchronization is enabled, the router has received synchronized routes from an IGP.

BGP bases it's decision first on whether a path is loop free, then on the policies indicated by the path attributes along with the policies configured on the router. The following summarized how BGP chooses the best path to a given destination.

1 If the next hop is not reachable through an IGP route installed in the routing table, do not consider this prefix for installation in the routing table.

If the only route you have to the next hop indicated in the NEXT_HOP attribute of a prefix is learned through iBGP, the route will oscillate in the routing table. It will be installed by BGP, then removed about 60 seconds later, only to be reinstalled momentarily after it is deleted.

2 If the path is internal, synchronization is enabled, and the route is not in the IGP, do not consider the route.

3 Prefer the path with the largest weight (weight is a Cisco proprietary parameter). The weight is generally used to prefer routes which are originated by this router over routes originated by other routers.

4 If the routes have the same weight, prefer the route with the largest local preference.

For example, a route might be originated by the local router using the network (BGP) or aggregate-address command, or through redistribution from an IGP. BGP prefers local routes originated by network (BGP) and redistribute commands over local aggregates originated by the aggregate-address command.

5 If the local preference is the same, or if no route was originated by the local router, prefer the route with the shortest autonomous system path. Also note the following:

• BGP skips this step if the bgp bestpath as-path ignore command is configured.

• No matter how many autonomous systems are in a set, an autonomous system set counts as one.

• The autonomous system confederation sequence is not included in the autonomous system path length.

6 If the autonomous system path length is the same, prefer the route with the lowest origin code (IGP < EGP < INCOMPLETE).

7 If the origin codes are the same, prefer the route with the lowest Multi Exit Discriminator (MED) metric attribute.

A comparison is only done if the neighboring autonomous system is the same for all routes considered. Also note the following:

• If the bgp always-compare-med command is enabled, BGP compares the MED for routes from neighbors in different autonomous systems. Also, if this command is enabled, it must be enabled throughout the autonomous system; otherwise, routing loops can occur.

• If the bgp bestpath med-confed command is enabled, the MED is compared for all routes that are originated within a local confederation.

• BGP will change the MED of a route received from a neighbor with a value of infinity to a value of one less than infinity before the route is inserted into the BGP table.

• The most recent IETF decision regarding BGP MED assigns a value of infinity to a missing MED, making the route lacking the MED variable the least preferred. The default behavior of BGP routers running Cisco IOS software is to treat routes without the MED attribute as having a MED of 0, making the route lacking the MED variable the most preferred. To configure the router to conform to the IETF standard, use the bgp bestpath missing-as-worst command.

• If the bgp deterministic med command is enabled, BGP compares the MED variable when choosing among routes advertised by the same sub-autonomous system within a confederation. It the bgp deterministic med command is disabled, the order in which routes are received may affect MED-based best path decisions.

8 Prefer the external (EBGP) route over the internal (IBGP) route.

All confederation routes are considered internal routes.

9 Prefer the route that can be reached through the closest IGP neighbor (the lowest IGP metric).

This means the router will prefer the shortest internal path within the autonomous system to reach the destination (the shortest path to the BGP next-hop).

10 If the following conditions are all true, insert the route for this path into the IP routing table:

• Both the best route and this route are external.

• Both the best route and this route are from the same neighboring autonomous system.

• The maximum-paths command is enabled.

11 If multipath is enabled, prefer the route that was received first (the oldest route).

This step minimizes route flap in that a newer route will not displace an older route even if the newer route is the preferred route based on the additional criteria discussed below.

If any of the following additional criteria are met, this step is skipped:

• The bgp bestpath compare-routerid command is enabled. If this command is enabled, BGP compares similar routes received from external BGP peers and selects the route with the lowest router ID.

• The router ID is the same for multiple routes, for example, the routes were received from the same router.

• No current best path exists, for example, a neighbor advertising the current best path has gone down.

12 If multipath is not enabled, prefer the route with the lowest IP address value for the BGP router ID.

The router ID is usually the highest IP address on the router or the loopback (virtual) address, but might be implementation-specific. You can configure a fixed router ID by using the bgp router-id command.

If a route contains route reflector attributes, the originator ID is substituted for the router ID in the route selection process.

13 If multipath is enabled and the originator or router ID is the same for multiple paths, prefer the path with the minimum cluster ID length.

The minimum cluster ID length attribute applies to BGP route reflector environments only.

14 Prefer the route coming from the lowest neighbor address.

The BGP neighbor configuration uses this IP address. The IP address corresponds to the remote peer used in the TCP connection with the local router.

Network Diagram

This document uses this network setup:

In that network diagram, 1.0.0.0/8 and 2.0.0.0/8 are advertised by AS 300 to the outside.

Configuration to Receive Full Internet Routing Table

The following configuration allows Router A to peer with BGP speakers in other autonomous systems. The route-map local only allows only the locally generated routes to be advertised to both of the service providers. In other words, they filter the Internet routes from one service provider that go back to the other service provider. This prevents the risk that your autonomous system will become a transit AS for Internet traffic.

Router A

Current configuration:

router bgp 300 network 1.0.0.0 network 2.0.0.0

neighbor 10.10.10.10 remote-as 100 neighbor 10.10.10.10 route-map localonly out

!--- Outgoing policy route-map that filters routes to service provider A (SP-A).

neighbor 20.20.20.20 remote-as 200 neighbor 20.20.20.20 route-map localonly out

!--- Outgoing policy route-map that filters routes to service provider B (SP-B).

end

This AS-Path access list only permits locally originated BGP routes:

ip as-path access-list 10 permit ^$

This is an example of a route map that uses that AS-Path access list to filter the routes advertised to the external neighbors in the service provider networks:

route-map localonly permit 10 match as-path 10

Configuration to Receive Directly-Connected Routes

Router A

Current configuration:

router bgp 300 network 1.0.0.0 network 2.0.0.0

neighbor 10.10.10.10 remote-as 100 neighbor 10.10.10.10 route-map localonly out

!--- Outgoing policy route-map that filters routes to SP-A.

neighbor 10.10.10.10 route-map as100only in

!--- Incoming policy route-map that filters routes from SP-A.

neighbor 20.20.20.20 remote-as 200 neighbor 20.20.20.20 route-map localonly out

!--- Outgoing policy route-map that filters routes to SP-B.

neighbor 20.20.20.20 route-map as200only in

!--- Incoming policy route-map that filters routes from SP-B.

end

Because you only want to accept routes that are directly connected to the service providers, you must filter the routes that they send to you, as well as the routes that you advertise. This access list and route map permit only locally originated routes; use it to filter outbound routing updates:

ip as-path access-list 10 permit ^$

route-map localonly permit 10 match as-path 10

This access list and route map filter out anything that is not sourced within the first service provider network; use it to filter the routes that are learned from service provider A (SP-A).

ip as-path access-list 20 permit ^100$route-map as100only permit 10 match as-path 20

This access list and route map filter out anything that is not sourced within the second service provider network; use it to filter the routes that are learned from service provider B (SP-B).

ip as-path access-list 30 permit ^200$

route-map as200only permit 10 match as-path 30

You also need two default routes that are distributed back into the rest of your network, one pointed to each of the service provider entry points:

ip route 0.0.0.0 0.0.0.0 10.10.10.10ip route 0.0.0.0 0.0.0.0 20.20.20.20

Configuration to Receive Default Routes Only

Router A

Current configuration:

router bgp 300 network 1.0.0.0 network 2.0.0.0

neighbor 10.10.10.10 remote-as 100 neighbor 10.10.10.10 route-map localonly out

!--- Outgoing policy route-map that filters routes to SP-A.

neighbor 10.10.10.10 prefix-list ABC in

neighbor 20.20.20.20 remote-as 200 neighbor 20.20.20.20 route-map localonly out

!--- Outgoing policy route-map that filters routes to SP-B.

neighbor 20.20.20.20 prefix-list ABC in

ip prefix-list ABC seq 5 permit 0.0.0.0/0

!--- Prefix list to allow only default route updates.

end

Because you want Router A to receive only default routes and no other networks from SP-A and SP-B, you must permit only the default route and deny all other BGP updates. Use this prefix list to allow only the default route update 0.0.0.0/0 and to deny all other BGP updates on Router A:

ip prefix-list ABC seq 5 permit 0.0.0.0/0

Apply that prefix list on the inbound updates on individual BGP neighbors in this way:

neighbor 10.10.10.10 prefix-list ABC inneighbor 20.20.20.20 prefix-list ABC in

Lecturer no # 18

Reading: C30.3Content:-

NAT Configuration

NAT

Need of NAT

With the popularity of internet there is main problem of depletion of IP Address because firstly IP addresses are only 32 bits so as result exhaustion of the address space so to remove this we use Classless addressing scheme which helped make better use of the address space, and IPv6 was created to ensure that we will never run out of addresses again. However, classless addressing has only slowed the consumption of the IPv4 address space, and IPv6 has taken years to develop and will require years more to deploy

o Increasing Cost of IP Addresses: As any resource grows scarce, it becomes more expensive. Even when IP addresses were available, it cost more to get a larger number from a service provider than a smaller number. It was desirable to conserve them not only for the sake of the Internet as a whole, but to save money.

o Growing Concerns over Security: As Internet use increased in the 1990s, more “bad guys” started using the network also. The more machines a company had directly connected to the Internet, the greater their potential exposure to security risks.

So The IP Network Address Translator (NAT) is designed to conserve IP addresses.

IP NAT Address Terminology

As its name clearly indicates, IP Network Address Translation is all about the translation of IP addresses. When datagrams pass between the private network of an organization and the public Internet, one or more of the addresses in these datagrams are changed by the NAT router. This translation means that every transaction in a NAT environment involves not just a source address and a destination address, but potentially multiple addresses for each of the source and destination.

NAT Address Terms Based on Device Location (Inside/Outside)

o Inside Address: Any device on the organization's private network that is using NAT is said to be on the inside network. Thus, any address that refers to a device on the local network in any form is called an inside address.

o Outside Address: The public internet—that is, everything outside the local network—is considered the outside network. Any address that refers to a public Internet device is an outside address.

Key Concept: In NAT, the terms inside and outside are used to identify the location of devices. Inside addresses refer to devices on the organization’s private network; outside addresses refer to devices on the public Internet.

NAT Address Terms Based on Datagram Location (Local/Global)

An inside device always has an inside address; an outside device always has an outside address. However, there are two different ways of addressing either an inside or an outside device, depending on in which part of the network the address appears in a datagram:

o Local Address: This term describes an address that appears in a datagram on the inside network, whether it refers to an inside or outside address.

o Global Address: This term describes an address that appears in a datagram on the outside network, again whether it refers to an inside or outside address.

Key Concept: In NAT, the terms local and global are used to indicate in what network a particular address appears. Local addresses are used on the organization’s private network (whether to refer to an inside device or an outside device); global addresses are used on the public Internet (again, whether referring to an inside or outside device).

IP NAT Static and Dynamic Address Mappings

NAT Working:-

NAT allows us to connect a private (inside) network to a public (outside) network such as the Internet, by using an address translation algorithm implemented in a router that connects the two. Each time a NAT router encounters an IP datagram that crosses the boundary between the two networks it must translate addresses as appropriate. But how does it know what to translate, and what to use for the translated address?

The NAT software in the router must maintain a translation table to tell it how to operate. The translation table contains information that maps the inside local addresses of internal devices (their regular addresses) to inside global address representations (the special public addresses used for external communication). It may also contain mappings between outside global addresses and outside local addresses for inbound transactions, if appropriate.

There are two basic ways that entries can be added to the NAT translation table.

Static Mappings

When static mappings are used, a permanent, fixed relationship is defined between a global and a local representation of the address of either an inside or an outside device. For example, we can use a static translation if we want the internal device with an inside local address of 10.0.0.207 to always use the inside global address of 194.54.21.10. Whenever 10.0.0.027 initiates a transaction with the Internet, the NAT router will replace that address with 194.54.21.10.

Dynamic Mappings

With dynamic mappings, global and local address representations are generated automatically by the NAT router, used as needed, and then discarded. The most common way that this is employed is in allowing a pool of inside global addresses to be shared by a large number of inside devices.

For example, say we were using dynamic mapping with a pool of inside global addresses available from 194.54.21.1 through 194.54.21.20. When 10.0.0.207 sent a request to the Internet it would not automatically have its source address replaced by 194.54.21.10. One of the 20 addresses in the pool would be chosen by the NAT router. The router would then watch for replies back using that address and translate them back to 10.0.0.207. When the session was completed, it would discard the entry to return the inside global address to the pool

IP NAT Unidirectional (Traditional/Outbound) Operation

Table 74: Operation Of Unidirectional (Traditional/Outbound) NAT

Step #

DescriptionDatagram Type

Datagram Source Address

Datagram Destination Address

1

Inside Client Generates Request And Sends To NAT Router: Device 10.0.0.207 generates an HTTP request that is eventually passed down to IP and encapsulated in an IP datagram. The source address is itself, 10.0.0.207, and the destination is 204.51.16.12. The datagram is sent to the NAT-capable router that connects the organization's internal network to the Internet.

Request (from inside client to outside server)

10.0.0.207(Inside Local)

204.51.16.12(Outside Local)

2

NAT Router Translates Source Address and Sends To Outside Server: The NAT router realizes that 10.0.0.207 is an inside local address and knows it must substitute an inside global address in order to let the public Internet destination respond. It consults its pool of addresses and sees the next available one is 194.54.21.11. It changes the source address in the datagram from 10.0.0.207 to 194.54.21.11. The destination address is not translated in traditional NAT. In other words, the outside local address and outside global address are the same.

The NAT router puts the mapping from 10.0.0.207 to 194.54.21.11 into its translation table. It sends out the modified datagram, which is eventually routed to the server at 204.51.16.12.

194.54.21.11(Inside Global)

204.51.16.12(Outside Global)

         

3

Outside Server Generates Response And Sends Back To NAT Router: The server at 204.51.16.12 generates an HTTP response. It of course has no idea that NAT was involved; it sees 194.54.21.11 in the request sent to it, so that's where it sends back the response. It is then routed back to the original client's NAT router.

Response (from outside server to inside client)

204.51.16.12(Outside Global)

194.54.21.11(Inside Global)

4 NAT Router Translates Destination Address And Delivers Datagram To Inside Client: The NAT router sees 194.54.21.11 in the response that arrived from the Internet. It consults its translation table and knows this datagram is intended for 10.0.0.207. This time the destination address is changed but not the source. It then delivers the

204.51.16.12(Outside Local)

10.0.0.207(Inside Local)

datagram back to the originating client.

IP NAT Bidirectional (Two-Way/Inbound) Operation

Traditional NAT is designed to handle only outbound transactions; clients on the local network initiate requests and devices on the Internet send back responses. However, in some circumstances, we may want to go in the opposite direction. That is, we may want to have a device on the outside network initiate a transaction with one on the inside. To permit this, we need a more capable type of NAT than the traditional version. This enhancement goes by various names, most commonly Bidirectional NAT, Two-Way NAT and Inbound NAT. All of these convey the concept that this kind of NAT allows both the type of transaction we saw in the previous topic and also transactions initiated from the outside network.The Problem with Inbound NAT: Hidden Addresses

Table 75: Operation Of Bidirectional (Two-Way/Inbound) NAT

Step # DescriptionDatagram Type

Datagram Source Address

Datagram Destination Address

1

Outside Client Generates Request And Sends To NAT Router: Device 204.51.16.12 generates a request to the inside server. It uses the inside global address 194.54.21.6 as the destination. The datagram will be routed to the local router for that address, which is the NAT router that services the inside network where the server is located.

Request (from outside client to inside server)

204.51.16.12(Outside Global)

194.54.21.6(Inside Global)

2

NAT Router Translates Destination Address and Sends To Inside Server: The NAT router already has a mapping from the inside global address to the inside local address of the server. It replaces the 194.54.21.6 destination address with 10.0.0.207, and performs checksum recalculations and other work as necessary. The source address is not translated. The router then delivers the modified datagram to the inside server at 10.0.0.207.

204.51.16.12(Outside Local)

10.0.0.207(Inside Local)

         

3

Inside Server Generates Response And Sends Back To NAT Router: The server at 10.0.0.207 generates a response, which it addresses to 204.51.16.12 since that was the source of the request to it. This is then routed to the server's NAT router.

Response (from inside server to outside client)

10.0.0.207(Inside Local)

204.51.16.12(Outside Local)

4

NAT Router Translates Source Address And Routes Datagram To Outside Client: The NAT router sees the private address 10.0.0.207 in the response and replaces it with 194.54.21.6. It then routes this back to the original client on the outside network.

194.54.21.6(Inside Global)

204.51.16.12(Outside Global)

IP NAT Port-Based ("Overloaded") Operation: Network Address Port Translation (NAPT)/PAT

Now, let's come back to NAT. We are already translating IP addresses as we send datagrams between the public and private portions of the internetwork. What if we could also translate port numbers? Well, we can! The combination of an address and port uniquely identifies a connection. As a datagram passes from the private network to the public one, we can change not just the IP address but also the port number in the TCP or UDP header. The datagram will be sent out with a different source address and port. The response will come back to this same address and port combination (called a socket) and can be translated back again.

Port-based NAT of course requires a router that is programmed to make the appropriate address and port mappings for datagrams as it transfers them between networks. The disadvantages of the method include this greater complexity, and also more potential compatibility issues (such as with applications like FTP) since we must now watch for port numbers at higher layers and not just IP addresses.

Port-based or “overloaded” NAT is an enhancement of regular NAT that allows a large number of devices on a private network to simultaneously “share” a single inside global address by changing the port numbers used in TCP and UDP messages

Table 76: Operation Of Port-Based (“Overloaded”) NAT

Step #

DescriptionDatagram Type

Datagram Source Address:Port

Datagram Destination Address:Port

1 Inside Client Generates Request And Sends To NAT Router: Device 10.0.0.207 generates an HTTP request to the server at 204.51.16.12. The standard server port for WWW is 80, so the destination port of the request is 80; let's say the source port on the client is 7000.

Request (from inside client to outside server)

10.0.0.207:7000(Inside Local)

204.51.16.12:80(Outside Local)

The datagram is sent to the NAT-capable router that connects the organization's internal network to the Internet.

2

NAT Router Translates Source Address And Port And Sends To Outside Server: The NAT router realizes that 10.0.0.207 is an inside local address and knows it must substitute an inside global address. Here though, there are multiple hosts sharing the single inside global address 194.54.21.7. Lets say that port 7000 is already in use for that address by another private host connection. The router substitutes the inside global address and also chooses a new source port number, say 7224, for this request. The destination address and port are not changed.

 

The NAT router puts the address and port mapping into its translation table. It sends the modified datagram out, which arrives at the server at 204.51.16.12.

194.54.21.7:7224(Inside Global)

204.51.16.12(Outside Global)

         

3

Outside Server Generates Response And Sends Back To NAT Router: The server at 204.51.16.12 generates an HTTP response. It of course has no idea that NAT was involved; it sees an address of 194.54.21.7 and port of 7224 in the request sent to it, so it sends back to that address and port.

Response (from outside server to inside client)

204.51.16.12:80(Outside Global)

194.54.21.7:7224(Inside Global)

4

NAT Router Translates Destination Address And Port And Delivers Datagram To Inside Client: The NAT router sees the address 94.54.21.7 and port 7224 in the response that arrived from the Internet. It consults its translation table and knows this datagram is intended for 10.0.0.207, port 7000. This time the destination address and port are changed but not the source. The router then delivers the datagram back to the originating client.

204.51.16.12:80(Outside Local)

10.0.0.207:7000(Inside Local)

IP NAT "Overlapping" / "Twice NAT" OperationAll three of the versions of NAT discussed so far—traditional, bidirectional and port-based—are normally used to connect a network using private, non-routable addresses to the public Internet, which uses unique, registered, routable addresses. With these kinds of NAT, there will normally be no overlap between the address spaces of the inside and outside network, since the former are private and the latter public. This enables the NAT router to be able to immediately distinguish inside addresses from outside addresses just by looking at them.Cases With Overlapping Private and Public Address Blocks

There are circumstances however where there may indeed be an overlap between the addresses used for the inside network, and the addresses used for part of the outside network. Consider the following cases:

o Private Network To Private Network Connections: Our example network using 10.0.0.0 block addresses might want to connect to another network using the same method. This situation might occur if two corporations merge and happened to be using the same addressing scheme (and there aren't that many private IP blocks, so this isn't that uncommon).

o Invalid Assignment of Public Address Space To Private Network: Some networks might have been set up not using a designated private address block but rather a block containing valid Internet addresses. For example, suppose an administrator decided that the network he was setting up “would never be connected to the Internet” (ha!) and numbered the whole thing using 18.0.0.0 addresses, which belong to the Massachusetts Institute of Technology (MIT). Then later, this administrator's shortsightedness would backfire when the network did indeed need to be connected to the 'net.

o “Stale” Public Address Assignment: Company A might have been using a particular address block for years that was reassigned or reallocated for whatever reason to company B. Company A might not want to go through the hassle of renumbering their network, and would then keep their addresses even while Company B started using them on the Internet.

Table 77: Operation Of “Overlapping” NAT / “Twice NAT”

Step #

DescriptionDatagram Type

Datagram Source Address

Datagram Destination Address

1

Inside Client Generates Request And Sends To NAT Router: Device 18.0.0.18 generates a request using the destination 172.16.44.55 that it got from the (NAT-intercepted) DNS query for “www.twicenat.mit.edu. The datagram is sent to the NAT router for the local network.

Request (from inside client to outside server)

18.0.0.18(Inside Local)

172.16.44.55(Outside Local)

2

NAT Router Translates Source Address And Destination Address and Sends To Outside Server: The NAT router makes two translations. First, it substitutes the 18.0.0.18 address with a publicly registered address, which is 194.54.21.12 for this example. It then translates the bogus 172.16.44.55 back to the real MIT address for “www.twicenat.mit.edu”. It routes the datagram to the outside server.

194.54.21.12(Inside Global)

18.1.2.3(Outside Global)

         

3

Outside Server Generates Response And Sends Back To NAT Router: The MIT server at 18.1.2.3 generates a response and sends it back to 194.54.21.12, which causes it to arrive back at the NAT router.

Response (from outside server to inside client)

18.1.2.3(Outside Global)

194.54.21.12(Inside Global)

4

NAT Router Translates Source Address And Destination Address And Delivers Datagram To Inside Client: The NAT router translates back the destination address to the actual address being used for our inside client, as in regular NAT. It also substitutes back in the 172.16.44.55 value it is using as a substitute for the real address of “www.twicenat.mit.edu”.

172.16.44.55(Outside Local)

18.0.0.18(Inside Local)

Configuring an NAT router

To configure an NAT router, do the following.1. To specify the public IP address pool ranging from first IP to last IP, use the following Global Configuration command:ip nat pool name of pool first IP last IP netmask mask

2. To define an access list controlling which internal hosts can use the IP addresses in the pool, use the following Global Configuration command:access-list access-list number deny host denied host IPaccess-list access-list number permit network address bit maskThe access-list number parameter in the above commands represents an IP standard access-list, with valid values ranging from 0 to 99. The bit mask parameter specifies which bits in the network address should be ignored. A “1” (“0”) in the bit mask means the corresponding network address bit should be ignored (compared).

3. Associate the access-list with the public IP address pool:ip nat inside source list access-list number pool name of pool.

4. To specify a router interface which has a public IP address and connects to the Internet, use the following Interface Configuration commands:interface name of interfaceip address public IP address netmaskip nat outside

5. To specify a router interface which has a private IP address and connects to the private network, use the following Interface Configuration commands:interface name of interfaceip address private IP address netmaskip nat inside

6. To define a static translation, use:

ip nat inside source static private IP address public IP address Note that if a static translation is defined, the internal host with the private IP address should be denied from using the shared public address pool.

7. To configure PAT, use:ip nat inside source list list number interface \ router interfaceoverloadThen all the internal hosts use the same public IP address, i.e., the IP address of the outside router interface, using port translations.

Lecture no # 19Reading: www.linuxhomenetworking.comContent:Configuring windows box / Linux box as router

Configuring Linux box as router Prerequisite:1. It needs atleast 2 Network cards2. enable IP_Farwarding3. Define required gatewaysenable IP_FarwardingIP_Forwarding can be activated by two ways.1. Include following line in /etc/sysconfig/network fileIP_FARWARD= YESOr2. By appending following line in /etc/rc.local fileecho "1" > /proc/sys/net/ipv4/ip_forwardDefine required gateways:Required gateway can be define in /etc/rc.local file. To define a specific gateway appendfollowing line in /etc/rc.local file:/sbin/route add -net 172.27.0.0 netmask 255.255.240.0 gw 172.27.31.254Above line need three parameters, network address, subnet mask and gateway address of other network.To define a specific gateway append following line in /etc/rc.local file:/sbin/route add -net default gw 172.31.127.254Sample File:/ete/rc.local#!/bin/sh## This script will be executed *after* all the other init scripts.# You can put your own initialization stuff in here if you don't# want to do the full Sys V style init stuff.# to enable IPv4 forwarding#/sbin/route add -net 202.141.40.0 netmask 255.255.255.0 gw 172.31.44.1#/sbin/route add -net 172.27.16.0 netmask 255.255.240.0 gw 172.31.127.252echo "1" > /proc/sys/net/ipv4/ip_forward# default route for outside world/sbin/route add -net default gw 172.31.127.254# route for the security network/sbin/route add -net 172.27.0.0 netmask 255.255.240.0 gw 172.27.31.254# route for the home pcs (22 Aug 2003) (Not Required)#/sbin/route add -net 172.30.0.0 netmask 255.255.0.0 gw 172.31.11.104#echo "1" > /proc/sys/net/ipv4/conf/eth1/proxy_arp# Solution for “Network Table Overflow” error# increase ARP cache sizes# default kernel values are 1024, 512, 128echo 8192 > /proc/sys/net/ipv4/neigh/default/gc_thresh3echo 4096 > /proc/sys/net/ipv4/neigh/default/gc_thresh2echo 1024 > /proc/sys/net/ipv4/neigh/default/gc_thresh1Troubleshooting:If you get “Network Table Overflow” error, it means default arp table cache size (1024) is not sufficient for your router. At any point of time, more than 1024 machines are trying to use the router. So increase arp table threshold values by adding following lines in /etc/rc.local file. Here in the following example about 5000 systems are on the network, so we chose an upper threshold value as 8192.echo 8192 > /proc/sys/net/ipv4/neigh/default/gc_thresh3echo 4096 > /proc/sys/net/ipv4/neigh/default/gc_thresh2

echo 1024 > /proc/sys/net/ipv4/neigh/default/gc_thresh1

Configuring windows box as router

1) get your access provider to route a block of addresses to you2) configure your RAS connection 3) configure TCP/IP settings for your network card. Leave the default gateway blank

By default, Windows can't forward incoming IP address, as a result it can't route IP address between networks.

But we could make Windows as a PC router by adding little modification on the registry.

4) edit registry settings on your router machine to add values as follows:

HKEY_LOCAL_MACHINE \System\CurrentControlSet\Services\RasArp\Parameters\DisableOtherSrcPackets Data type REG_DWORD, value = 0HKEY_LOCAL_MACHINE \System\CurrentControlSet\Services\Tcpip\Parameters\IPEnableRouter Data type REG_DWORD, value = 15) On other machines on your LAN, set gateway to the IP of the machine used as the router.6) NOTE: if you have a recent NT Service Pack installed, you must have the IP addresses of your LAN on a different subnet than your incoming RAS connection. For instance, let's say your service provider routes packets to address xxx.xxx.xxx.1 (your incoming RAS connection on the router PC). Configure the Ethernet card on that PC to be xxx.xxx.xxx.129 and use a Subnet Mask of 255.255.255.128. Give the other PCs on your LAN IP addresses above 129 and the same subnet mask. 7. Restart windowsYou are all set. When the router machine is online, dialed into your access provider, it will route IP packets to and from any other machine on your network

Note:On Windows 2000/NT we don't need to modify the registry because there is an option to make windows as PC router

Enter control panel> network > TCP/IP Properties > router > IP Forwarding

Lecture no # 20Reading: G23.8 Content:Dialup configuration and Authentication: PPP

Point-to-Point Protocol

The Point-to-Point Protocol (PPP) originally emerged as an encapsulation protocol for transporting IP traffic over point-to-point links. PPP also established a standard for the assignment and management ofIP addresses, asynchronous (start/stop) and bit-oriented synchronous encapsulation, network protocol multiplexing, link configuration, link quality testing, error detection, and option negotiation for such capabilities as network layer address negotiation and data-compression negotiation. PPP supports these functions by providing an extensible Link Control Protocol (LCP) and a family of Network Control Protocols (NCPs) to negotiate optional configuration parameters and facilities. PPP ComponentsPPP provides a method for transmitting datagrams over serial point-to-point links. PPP contains three main components:• A method for encapsulating datagrams over serial links. PPP uses the High-Level Data Link Control(HDLC) protocol as a basis for encapsulating datagrams over point-to-point links. • An extensible LCP to establish, configure, and test the data link connection.• A family of NCPs for establishing and configuring different network layer protocols. PPP is designed to allow the simultaneous use of multiple network layer protocols.General OperationTo establish communications over a point-to-point link, the originating PPP first sends LCP frames to configure and (optionally) test the data link. After the link has been established and optional facilities have been negotiated as needed by the LCP, the originating PPP sends NCP frames to choose and configure one or more network layer protocols. When each of the chosen network layer protocols has been configured, packets from each network layer protocol can be sent over the link. The link will remain configured for communications until explicit LCP or NCP frames close the link, or until some external event occurs (for example, an inactivity timer expires or a user intervenes).PPP Link LayerPPP uses the principles, terminology, and frame structure of the International Organization forStandardization (ISO) HDLC procedures (ISO 3309-1979), as modified by ISO 3309:1984/PDAD1“Addendum 1: Start/Stop Transmission.” ISO 3309-1979 specifies the HDLC frame structure for use in synchronous environments. ISO 3309:1984/PDAD1 specifies proposed modifications to ISO 3309-1979 to allow its use in asynchronous environments. The PPP control procedures use the definitions and control field encodings standardized in ISO 4335-1979 and ISO 4335-1979/Addendum 1-1979. The PPP frame format appears in Figure 13-1.

The following descriptions summarize the PPP frame fields illustrated in Figure 13-1:• Flag—A single byte that indicates the beginning or end of a frame. The flag field consists of the binary sequence 01111110.• Address—A single byte that contains the binary sequence 11111111, the standard broadcast address. PPP does not assign individual station addresses.Control—A single byte that contains the binary sequence 00000011, which calls for transmission of user data in an unsequenced frame. A connectionless link service similar to that of Logical LinkControl (LLC) Type 1 is provided.

• Protocol—Two bytes that identify the protocol encapsulated in the information field of the frame.The most up-to-date values of the protocol field are specified in the most recent Assigned NumbersRequest For Comments (RFC).• Data—Zero or more bytes that contain the datagram for the protocol specified in the protocol field.The end of the information field is found by locating the closing flag sequence and allowing 2 bytes for the FCS field. The default maximum length of the information field is 1,500 bytes. By prior agreement, consenting PPP implementations can use other values for the maximum information field length.• Frame check sequence (FCS)—normally 16 bits (2 bytes). By prior agreement, consenting PPP implementations can use a 32-bit (4-byte) FCS for improved error detection.The LCP can negotiate modifications to the standard PPP frame structure. Modified frames, however, always will be clearly distinguishable from standard frames.PPP Link-Control ProtocolThe PPP LCP provides a method of establishing, configuring, maintaining, and terminating the point-to-point connection. LCP goes through four distinct phases. First, link establishment and configuration negotiation occur. Before any network layer datagrams (for example, IP) can be exchanged, LCP first must open the connection and negotiate configuration parameters. This phase is complete when a configuration-acknowledgment frame has been both sent and received.This is followed by link quality determination. LCP allows an optional link quality determination phase following the link-establishment and configuration-negotiation phase. In this phase, the link is tested to determine whether the link quality is sufficient to bring up network layer protocols. This phase is optional. LCP can delay transmission of network layer protocol information until this phase is complete.At this point, network layer protocol configuration negotiation occurs. After LCP has finished the link quality determination phase, network layer protocols can be configured separately by the appropriateNCP and can be brought up and taken down at any time. If LCP closes the link, it informs the network layer protocols so that they can take appropriate action.Finally, link termination occurs. LCP can terminate the link at any time. This usually is done at the request of a user but can happen because of a physical event, such as the loss of carrier or the expiration of an idle-period timer.Three classes of LCP frames exist. Link-establishment frames are used to establish and configure a link.Link-termination frames are used to terminate a link, and link-maintenance frames are used to manage and debug a link.These frames are used to accomplish the work of each of the LCP phases.

PPP Configuration setup for serial TCP/IP phone or wireless connections between a local and remote Linux Box.

A. Phone connection

Note: It is a good idea to set up a basic dial-up connection and check that the serial link and mgetty program are functioning properly. Follow steps in HOWTO.2 and make sure the remote computer is sending a clean login prompt to your screen.

Setup procedure for PPP server (dial-in):

Step 1:File: /etc/inittabLine to add: d1:2345:respawn:/sbin/mgetty –D /dev/ttyS#

Where # is the number of the port which will be monitored by the mgetty process. This port should be dedicated to incoming calls and not be used to interface with any other devices (such as a UPS serial connection).

Note that the –D option is important as it forces mgetty to treat the modem as a DATA modem. No fax initialization is attempted.

Re-boot the machine and the mgetty process will be started automatically by the inittab master process.

Step 2:Using the user configuration panel, create a new user ppp. Set the password, user information, and create the /home/ppp directory. Do not make any changes to the default shell at this time. Close the configuration panel and activate the changes.

Edit the /etc/passwd file and replace the default shell with /usr/sbin/pppd. This is not a recognized shell by the user configuration control panel and this is why the /etc/passwd file has to be edited separately. After the ppp login authentication process has completed, the remote server will start the pppd process automatically instead of the normal shell.

Note: Do not edit the /etc/passwd file to create the ppp account. Edit the file only to modify the shell, after the account has been created through the control panel.

Step 3:Create the file .ppprc in /home/ppp and add the following lines

-detachmodemlockcrtsctsproxyarplocalhostIP:remotehostIP

Note: If the PPP server is networked, then it should already have an IP address, and you must replace the string localhostIP with it. If it is going to be used as a stand-alone machine, then make up a dummy IP address. Replace the string remotehostIP with the IP address of the PPP client. If the PPP client calling the server already has a static IP address on some remote network, make sure the dummy IP address assigned to the server will not conflict with another valid IP on the PPP client’s network. The server may allow clients with different IP addresses to dial in by adding more lines of the form localhostIP:remotehostIP to the .ppprc file.If you wish to enable any client to establish a PPP connection with the server, do not include the address of the PPP client in line 6 of the .ppprc file. Only use the string localhostIP:

Step 4:File: /etc/rc.d/rc.localLine 1 to add: chmod u+s /usr/sbin/pppdLine 2 to add: chmod a+rw /dev/ttyS*This gives system permission to any logged user to run the pppd daemon and ensures that the device are accessible by everyone.

Step 5:If your PPP server is a Linux box on the local Ethernet, and you want your standalone PPP client to be able to see machines behind the server (i.e. you can ping any valid IP address), you must enable IP forwarding by the server. Edit the file /etc/sysconfig/network and change the line that says FORWARD_IPV4=no, to FORWARD_IPV4=yes. This is absolutely essential for a seamless connection to the internet. The other parameter of importance is the proxyarp option above (which sets up a proxy ARP entry in the server’s ARP table which says ‘send all packets destined to the PPP client to me’. This is the easiest way to set up routing to a single PPP client.

Setup procedure for PPP client (dial-out):

Step 1:

Copy the chat scripts from /usr/docs/ppp.SOMEVERSION/scripts to the user directory. You may create a separate ppp directory for the scripts.

The only scripts that are needed for a standard ppp connection are ppp-on, ppp-on-dialer, and ppp-off. Make sure that all three files are executable by issuing the command chmod a+x <file> for each file.

Edit the file ppp-on and make the required changes to the lines shown below:

TELEPHONE=telephone number of PPP serverACCOUNT=pppPASSWORD=ppp12345LOCAL_IP=xxx.xxx.xxx.xxxREMOTE_IP=xxx.xxx.xxx.xxxNETMASK=255.255.255.0

DIALER_SCRIPT=/ppp_scripts_directory/ppp-on-dialer

exec /usr/sbin/pppd debug lock modem crtcts /dev/ttyS# 19200 \$LOCAL_IP:$REMOTE_IP \netmask $NETMASK defaultroute connect $DIALER_SCRIPT

Notes:The shell variable REMOTE_IP is the IP address of the dial-in PPP server. If the server was set up to allow any dial-in connections, then leave the IP address blank (i.e. REMOTE_IP= ) in the connection setup parameters, and leave out the $REMOTE_IP variable from the command exec (i.e. $LOCAL_IP: ).The defaultroute parameter adds a default route to the client’s routing system. If the PPP client is establishing a connection to a networked PPP server, and you want to be able to see machines beyond the server, the IP address assigned to the client should belong to the same subnet as that of the server (since we are using a netmask of 255.255.255.0). Choose a number between 1 and 255 that is not already assigned to a machine on the server’s subnet.

Step2:Issue the command chmod u+s /usr/sbin/pppdThis gives system permission to any logged user to run the pppd daemon.

Step3:

Modify the ppp-on-dialer file to conform with the “chat” strings exchanged by the local modem – remote modem and computer. Each line of the dialer script consists of an “expect string” “send string” pair.

Note: The script below (default template) will work without modification for a U.S. Robotics Courier V. Everything modem connecting to a PPP server running RedHat Linux 5.2. Note that the “expect string” consists of the standard “login” string prompt sent by the remote computer. Keep in mind that if the client connects to a server such as an Internet Service provider, the “expect string” may be different. The remote server may be sending a “username” string instead of the “login” string”. Other “expect strings” sent by the remote server may have to be inserted in the ppp-on-dialer script as well. The easiest way to determine which strings are sent by your provider is to use the Linux communication program cu to call the provider directly, and record the strings echoed to the screen by the remote computer, as well as the ones you have to type in, all the way to the password prompt.

#!/bin/sh## This is part 2 of the ppp-on script. It will perform the connection# protocol for the desired connection.#

exec chat -v \TIMEOUT 3 \ABORT '\nBUSY\r' \ABORT '\nNO ANSWER\r' \ABORT '\nRINGING\r\n\r\nRINGING\r' \'' \rAT \'OK-+++\c-OK' ATH0 \TIMEOUT 30 \OK ATDT$TELEPHONE \CONNECT '' \ogin:--ogin: $ACCOUNT \assword: $PASSWORD

Initiating the connection:

From the PPP client, issue the command ./ppp-on to invoke the scriptThe connection process can be monitored on the PPP client by opening another xterm and typingtail –f /var/log/messages to see all the diagnostic messages sent by the client’s PPP daemon.On the server side, the same command can be used to capture the messages sent by its own daemon once it gets started. Additionally, tail –f /var/log/mgetty.log/ttySx can be invoked to check on the status of the serial connection itself.

To confirm that a valid PPP connection exists, type ifconfig on the client to see the ppp0 network interface and relation information in addition to the lo (local host) network interface. Pinging the remote server shall instill further confidence that the connection has been made successfully.

B. Wireless connection

Note: It is a good idea to set up a basic connection and check the serial link with the mgetty program running on the server. Follow steps in HOWTO.2 for the wireless connection and make sure the remote computer is sending a clean login prompt to your screen.

Setup procedure for PPP server :

Once you have tested connectivity with the server running the mgetty daemon on the serial port, you can replace the mgetty program with the pppd daemon in the /etc/inittab file

File: /etc/inittabLine to add: d1:2345:respawn:/sbin/pppd –detach lock crtscts /dev/ttyS# <LOCAL_IP>:<REMOTE_IP> <speed>

where<LOCAL_IP> is the IP address of the server. <REMOTE_IP> is the IP address of the PPP client. If you wish to enable any client to establish a PPP connection with the server, leave this field blank.<speed> is the connect speed desired.

Re-boot the machine and the pppd process will be started automatically by the inittab master process.

The defaultroute parameter adds a default route to the client’s routing system. If the PPP client is establishing a connection to a networked PPP server, and you want to be able to see machines beyond the server, the IP address assigned to the client should belong to the same subnet as that of the server (since we

are using a netmask of 255.255.255.0). Choose a number between 1 and 255 that is not already assigned to a machine on the server’s subnet.

Where # is the number of the port which will be monitored by the mgetty process. This port should be dedicated to incoming calls and not be used to interface with any other devices (such as dial-out modem, or UPS serial connections).

Setup procedure for PPP client:

When you are ready to establish a PPP connection with the server (you can automate this with a cron job), issue the command Once you have tested connectivity with the server running the mgetty daemon on the serial port, you can replace the mgetty program with the pppd daemon in the /etc/inittab file

/sbin/pppd –detach crtscts lock /dev/ttyS# <LOCAL_IP>:<REMOTE_IP> <speed> &

This command should be run in the background and your connect speed should match the setting on the PPP server. You do not need to validate the connection using username/password pairs as for a dialup connection since you have physical control of both machines.

Initiating the connection:

As outlined above, as soon as pppd is initiated on the client, it will bring up the link and you have access to the standard TCP/IP application programs. The connection process can be monitored on the PPP client by opening another xterm and typingtail –f /var/log/messages to see all the diagnostic messages sent by the client’s PPP daemon.On the server side, the same command can be used to capture the messages sent by its own daemon once the connection is established. To confirm that a valid PPP connection exists, type ifconfig on the client to see the ppp0 network interface and relation information in addition to the lo (local host) network interface. Pinging the remote server shall instill further confidence that the connection has been made successfully.

Note:

The pppd command with the above options will bring up the link between two non-networked computers. No routing as been specified yet. If the PPP server is connected to a local network, you should add the command-line option proxyarp to pppd started by the inittab process. This option sets up a proxy ARP entry in the server’s ARP table which says ‘send all packets destined to the PPP client to me’. This is the easiest way to set up routing to a single PPP client. Furthermore, if you want your standalone PPP client to be able to see machines behind the server (i.e. you can ping any valid IP address), you must enable IP forwarding by the server. Edit the file /etc/sysconfig/network and change the line that says FORWARD_IPV4=no, to FORWARD_IPV4=yes. This is absolutely essential for a seamless connection to the internet.

On the client side, you must add the option defaultroute to the pppd command. The defaultroute parameter adds a default route to the client’s routing system. Also, if the PPP client is establishing a connection to a networked PPP server, and you want to be able to see machines beyond the server, the IP address assigned to the client should belong to the same subnet as that of the server (since we are using a netmask of 255.255.255.0). Choose a number between 1 and 255 that is not already assigned to a machine on the server’s subnet. If the PPP client calling the server already has a static IP address on some remote network, make sure the dummy IP address assigned to the server will not conflict with another valid IP on the PPP client’s network.

Lecture no # 21Reading: http://en.wikipedia.org/wiki/RADIUS ,http://en.wikipedia.org/wiki/Remote_Access_ServiceContent:Radius, RAS

RADIUS Overview

RADIUS is a distributed client/server system that secures networks against unauthorized access. In the Cisco implementation, RADIUS clients run on Cisco routers and send authentication requests to a central RADIUS server that contains all user authentication and network service access information. RADIUS is a fully open protocol, distributed in source code format that can be modified to work with any security system currently available on the market. Cisco supports RADIUS under its AAA security paradigm. RADIUS can be used with other AAA security protocols, such as TACACS+, Kerberos, or local username lookup. RADIUS is supported on all Cisco platforms.

RADIUS has been implemented in a variety of network environments that require high levels of security while maintaining network access for remote users.

Use RADIUS in the following network environments that require access security:

• Networks with multiple-vendor access servers, each supporting RADIUS. For example, access servers from several vendors use a single RADIUS server-based security database. In an IP-based network with multiple vendors' access servers, dial-in users are authenticated through a RADIUS server that has been customized to work with the Kerberos security system.

• Turnkey network security environments in which applications support the RADIUS protocol, such as in an access environment that uses a "smart card" access control system. In one case, RADIUS has been used with Enigma's security cards to validate users and grant access to network resources.

• Networks already using RADIUS. You can add a Cisco router with RADIUS to the network. This might be the first step when you make a transition to a Terminal Access Controller Access Control System (TACACS+) server.

• Networks in which a user must only access a single service. Using RADIUS, you can control user access to a single host, to a single utility such as Telnet, or to a single protocol such as Point-to-Point Protocol (PPP). For example, when a user logs in, RADIUS identifies this user as having authorization to run PPP using IP address 10.2.3.4 and the defined access list is started.

• Networks that require resource accounting. You can use RADIUS accounting independent of RADIUS authentication or authorization. The RADIUS accounting functions allow data to be sent at the start and end of services, indicating the amount of resources (such as time, packets, bytes, and so on) used during the session. An Internet service provider (ISP) might use a freeware-based version of RADIUS access control and accounting software to meet special security and billing needs.

RADIUS is not suitable in the following network security situations:

• Multiprotocol access environments. RADIUS does not support the following protocols:

• AppleTalk Remote Access (ARA) Protocol

• NetBIOS Frame Control Protocol (NBFCP)

• Router-to-router situations. RADIUS does not provide two-way authentication. RADIUS can be used to authenticate from one router to a non-Cisco router if the non-Cisco router requires RADIUS authentication.

• Networks using a variety of services. RADIUS generally binds a user to one service model.

RADIUS Operation

When a user attempts to log in and authenticate to an access server using RADIUS, the following steps occur:

1 The user is prompted for and enters a username and password.

2 The username and encrypted password are sent over the network to the RADIUS server.

3 The user receives one of the following responses from the RADIUS server:

(a) ACCEPT—The user is authenticated.

(b) REJECT—The user is not authenticated and is prompted to reenter the username and password, or access is denied.

(c) CHALLENGE—A challenge is issued by the RADIUS server. The challenge collects additional data from the user.

(d) CHANGE PASSWORD—A request is issued by the RADIUS server, asking the user to select a new password.

The ACCEPT or REJECT response is bundled with additional data that is used for EXEC or network authorization. You must first complete RADIUS authentication before using RADIUS authorization. The additional data included with the ACCEPT or REJECT packets consists of the following:

• Services that the user can access, including Telnet, rlogin, or local-area transport (LAT) connections, and PPP, Serial Line Internet Protocol (SLIP), or EXEC services.

• Connection parameters, including the host or client IP address, access list, and user timeouts.

RADIUS Configuration Task List

To configure RADIUS on your Cisco router or access server, you must perform the following tasks:

• Use the aaa new-model global configuration command to enable AAA. AAA must be configured if you plan to use RADIUS. For more information about using the aaa new-model command, refer to the "AAA Overview" chapter.

• Use the aaa authentication global configuration command to define method lists for RADIUS authentication. For more information about using the aaa authentication command, refer to the "Configuring Authentication" chapter.

• Use line and interface commands to enable the defined method lists to be used. For more information, refer to the "Configuring Authentication" chapter.

The following configuration tasks are optional:

• If needed, use the aaa authorization global command to authorize specific user functions. For more information about using the aaa authorization command, refer to the "Configuring Authorization" chapter.

• If needed, use the aaa accounting command to enable accounting for RADIUS connections. For more information about using the aaa accounting command, refer to the "Configuring Accounting" chapter.

Configure Router to RADIUS Server Communication

The RADIUS host is normally a multiuser system running RADIUS server software from Livingston, Merit, Microsoft, or another software provider. A RADIUS server and a Cisco router use a shared secret text string to encrypt passwords and exchange responses.

To configure RADIUS to use the AAA security commands, you must specify the host running the RADIUS server daemon and a secret text string that it shares with the router. Use the radius-server commands to specify the RADIUS server host and a secret text string.

To specify a RADIUS server host and shared secret text string, use the following commands in global configuration mode:

Step Command Purpose

1 radius-server host {hostname | ip-address} [auth-port port-number] [acct-port port-number]

Specify the IP address or host name of the remote RADIUS server host and assigns authentication and accounting destination port numbers.

2 radius-server key string Specify the shared secret text string used between the router and the RADIUS server.

To customize communication between the router and the RADIUS server, use the following optional radius-server global configuration commands:

Step Command Purpose

1 radius-server retransmit retries

Specify the number of times the router transmits each RADIUS request to the server before giving up (default is three).

2 radius-server timeout seconds

Specify the number of seconds a router waits for a reply to a RADIUS request before retransmitting the request.

3 radius-server deadtime minutes

Specify the number of minutes a RADIUS server, which is not responding to authentication requests, is passed over by requests for RADIUS authentication.

Lecture no #22

Reading:-A8.5, G24.1-24.6, C18.1-18.9, B202-266Contents:-Configuring a DNS Server

Configuring a DNS Server (Linux)

It uses BIND software (Berkeley internet name domain) which maintains the DNS related software suite that runs under Linux.

Step 1# check whether BIND RPM is installed or not# rpm –qa | grep bind*To check software packages are installed or not such as (bind utils,bind server etc)(if rpm id not installed ,install it)# mount/mnt/cdrom# cd/mnt/cdrom# cd/Redhat/RPMSInstall the packages using the following command # rpm –ivh bind*]# rpm –ivh cache*

Step 2# cd/etc/named.conf(DNS configuration file)Step3# service named startStep4# vi/etc/resolv.conf

Make entry here

Kamla(server name) 192.168.30.26(server ip):wq

Step5# vi/etc/sysconfig/network Make entry hereHOSTNAME ---- dear (computer name):wq

Step6# vi/etc/hosts

Make entry here

192.168.30.26 dear (computer name) local host local domain127.0.0.1 dear (computer name) local host local domain :wq

Step7# sysctl –w kernel hostname= “dear”

Step8# vi /etc/named.conf

In this file we have to make master configuration entry after following entry

Forwrders{192.168.30.26;};Forward only;}};Write this entryZone “Kamla.com”IN{Type master;File “Kamla”;};:wq

Step 9# service named restart

Step 10# cd/var/named/chroot/var/namedStep 11# lsStep 12 # cp localhost.zone KamlaStep 13 # vi Kamla View content of Kamla fileSTTl 86400@ IN SOA dear.Kamla.com.root.Kamla.com (File entry 42 serial 3n refresh 1sm retry 1w expire 1D minimu,

IN MX S Kamla.com IN NS dear.Kamla.comDear IN A 192.168.30.26Mail IN A 192.168.30.26:wq

Step 14# service named restartStep 15# namedStep16# named – checkconfStep17# service named startStep18# service named onStep19# named-checkzone Kamla.com kamla

Step20# vi/etc/named.conf

Last add this fileZone “30.168.192.in-addr-arpa”IN{Type master;File “rev”};:wq

Step 21# lsStep 22# cp local domain.zone revStep23# vi rev

30.168.192.IN-addr-arpa.IN SOA dear@Kamla.com.root.Kamla.com(Same entry)Write this entry IN NS dear.Kamla.com IN MX S Kamla.com26 IN PTR dear.Kamla.com59 IN PTR mail.Kamla.com:wq

Step 24# named-checkzone 30.168.192.IN-addr.arpa revStep25# service named restsrtStep26# host desr.Kamla.com (run)Step 27# host 192.168.30.26 (run)

To make slave (in client system) Step1# vi/etc/resolv.conf(add ip address of master & server name here)Step2# service named restartStep 3# host 192.168.30.26Step4# vi /etc/named.conf

Change here Zone “Kamla.com”IN{Type slave;Masters{192.168.30.26;};File “’slaves/Kamla.bk;};Zone “30.168.192.IN-addr.arpa”IN{Type slave;Masters{192.168.30.26; };File “slaves/rev.bk”;};:wq

Step5# service named restartStep6 # lsStep7# cd slavesStep 8# ls

Lecture no #23

Reading: - A8.6, G24.7-24.10, B271-308Contents:-Configuring Send mail Server

Configuring Send mail Server (Linux)

Step1# rpm –qa | grep “sendmail”Step2# cd/etc/mailStep3# lsStep4# vi sendmail.mc

In this file we have to editing so we have to uncomment this lines

Dnl# DAEMON-OPTIONS(port = SMTP , addr = 127.0.0.1 , Name- MTA)

We have to insert Dnl# to make uncomment/127 lines- this is line no in file we have to uncomment it:wq

Step5# m4 sendmail.mc > sendmail.cf

This will divert the changes made in sendmail.mc to sendmail.cf by this command

Step6# service sendmail restsrtStep7# lsStep 8# vi/etc/mail/access

In this file we make changes when we are creating a new server & want to send & receive mail from other network & give network ID of that particular network from which you wish to send or receive mail.

Local host local domain RelayLocal host Relay127.0.0.1 RelayIP address ( 192.168.30.48) or Allow / Deny192.168.30.0 /24:wq

Step9# vi virtuser table

Entry herexyz@abc.com mailxyz@rediffmail.com xy@gmail.com

step10# cd /

step 11# cd/etc/step 12# lsstep 13# vi aliases

Make entry here LIKERoot Ram, Sham

This mean that mail received by root are also received by all user shown above (Ram, Sham)

Step14# newaliases

It will show that entry are made in vi aliases

Step15# service sendmail restartStep16# telnet 192.168.30.26 25(port no)

Mail from : root@loacal host – sender OKRcpt to : ajay@local host- recipient OKData -------Clt+d to quit

Step17# to check mail on recipient side

Login- xxxxPassword ( root & user)

Step 18# mailStep 19# we have to enter mail no to see it details

Certificate assignment for POP3 & IMAP

POP3- post office protocolIMAP- internet message access protocol

These are two protocols which are specially used in mail server & email retrieves it

Step1# rpm –qa | grep “dovecot”Step 2# vi/etc/dovecot.conf

Make entry hereProtocol – imap imaps pop3 pop3 s(add) & uncomment it:wq

Step3# service dovecot restartStep4# chkconfig dovecot onStep5# nmap local host

This command is used to check the port for pop3 & pop3s are open or not

Step6# cd/usr/share/sslStep7# lsStep8# cd certsStep10# lsStep11# rm –rf dovecat.pemDelete this fileStep12# make dovecat.pem

Write entryCountry ---------orgState-------------org usedCity-------------osCompanyEmail:wq

Step13# ls-lStep14# cp dovecot.pem .. /private/

Overwrite private----y

Step15# service dovecat restartStep16# mutt-f {root@www.server(hostname).com}To retrive mail Or# mutt –f {192.168.30.26}

Yes checking mail

Step17# telnet 192.168.30.26

Mail from:root@192.168.30.26Recpt to: mail@192.168.30.26Data-----------Clt+d (quit)

Step18# to check mail on recipient sideLogon XXXPassword XXX (user /root)# mail

Type no to check that mail.

Lecture no #24

Reading: - A8.6, G24.13-24.15Contents:-Configuring a Web Server

Configuring a Web Server (Apache in Linux) Step1# rpm –qa | grep “httpd”Check apache packages such as apache –devel etcStep2# cd/etc/httpdStep3# cd confStep 4# vi httpd.conf

How to make html pagesStep5# cd /.Step6# cd/var/www/htmlStep 7# lsStep 8 # ls – aStep 9 # mkdir www.serverStep 10 # mv www.server serverStep 11 # lsStep 12# cd serverStep 13 # cat > ser.htmlThis is a text page of server from lab administsrtorStep 14 # pwdStep 15 # mv ser.html index.htmlStep 16# lsStep 17 # repeat step 4 Vi httpd.conf

Make entry here

Virtual host * 80 (port no) ip addres given here(host ip address)Server administsrator –Document root --/var/www/html/serverServer name – www.abc.com

Step 18# chkconfig httpd onStep 19# service httpd restartStep 20# hostnameStep 21# vi/etc/hosts

Make entry here

Ip system address –www.abc.com127.0.0.1 –same line

Step 22# vi/etc/sysconfig /networkMake entry here

Host – www.abc.com

Step 23# sysctl – w kernel.hostname=”www.abc.com

Step 24# service network restartStep 25# cd/var/www/html/serverStep 26# vi index.htmlStep 27# apachectl config_testStep 28# service httpd restart

Step 29 # elinks http:// 192.168.30.26 ( it will view text page)

Step 30# elinks http:// www.abc.com ( it will view text page)

Permission to open webpageStep1# cd/etc/httpd/confStep2# vi httpd.conf Make entry here<virtual hosts> copy hereDocument root--- Server name--- www.abc.com<directory /var/www/html/.server nameAllow override authconfigOrder allow,denyAllow 192.168.30.96—IP is allowed for userDeny 192.168.30.86 – it will deny user to view text page>

OrAllow /deny from <IP range></directory></virtual host>

Step3# service httpd restart

To create user

Step 4# cd/var/www/htmlStep 5# mkdir KamlaStep 6# cd KamlaStep 7# lsStep8# vi htaccess

Make entry hereAuthName “linux site”AuthType basicAuthUser file “etc/htpass”Require valid –user

Step 9# ls –aStep 10# htpasswd – mc /etc/htpass suu

Make entryPassword—TING

Step 11# htpasswd – m /etc/htpass suu

Make entry Password--- TING2

Step 12# cd /etc/ vi htpassIt will show user password in encrypted form.

Step 13# elinks http:// www.Kamla.com

C.G.I Script

Step 1# cd/var/www/cgi-binStep2# lsStep3# vi test.sh

Entry here

#/bin/bashEcho content-type:test/htmlEchoEcho “<Pre>”Echo my username isWho am iEchoEcho here is/etc/passwdCat/etc/passwdEchoEcho “</Pre>”:wq

Step4# chmod 777 test.shStep5# service httpd restartStep 6 # cd ..Step 7# cd/etc/httpd/confStep8# vi httpd.conf

Make entry hereVirtual host –Script alias /cgi-bin/ “var/www/cgi-bin/” Copy above line and paste below the server name in the file:wq

Step9# ls –aStep10# ! serStep11# apachectl configtestStep 12# elinks http:// www.Kamla.com /cgi-bin/test.sh

Certificate assignment to apache

Step1# rpm –qa |grep “ssl”Step2# cd/etc/httpd/confStep3# lsStep4# cd ssl.crtStep5# lsStep6# rm –rf server.crtStep7# cd ..Step8# cd ssl.keyStep9# cd ..Step10# makeStep11# make testcert

Write entryCountry –INState-HaryanaCity – GurgaonCompany-CDACOrg – CDTIOrg Unit-networkingComm. Name – linuxEmail- xyz@rediffmail.com

Step12# service httpd restartStep13# lsStep 14# cd ssl.crtStep15# lsStep16# cd..Step17# cd/etc/httpd/conf.dStep 18# lsStep19# vi ssl.conf

Make entry hereSsl engine onSsl certificate file/etc/httpd/conf/ssl.crt/abcCopy all these files

Step 20# vi/etc/httpd/conf/httpd.conf Write entry herePaste step 19 copy file here:wq

Step 21# apachectl config testStep 21# !sev (service httpd restart)Step 23# elinks https:// 192.168.30.26 (to see create certificate of webpage)

Lecture no #25

Reading: - www.microsoft.com , www.linuxhomenetworking.com.Contents:-Configuring a Proxy Server

Configuring a Proxy Server (squid Proxy)

Step1# rpm – qa | grep “squid”Step2# etc/squid/squid.conf Make entry here# NETWORK OPTIONS#http_port 3128http_port8080

Access control listsSrc-source client IP addressUrl_regex-URL regular expressionUrlpath_regex-Url PATH regular expression pattern matchingMaxconn-maximum numbers of connections limit from single client IP addressTime- Time of the day 7 day of week

# ACCESS CONTROLS-acl clients src 192.168.0.0 /255.255.255.0-http_access allow clients-http_access deny all

Maintain blacklist sitesAcl blacklist url_regex-I “/etc/squid/blacklist.txt”Blacklist will mean a group of all the url’s contained in the text file named blacklist.txtAcl blackpath urlpath_regex-I “/etc/squid/blackpath.txt”Blackpath will mean a group of the url’s in which the certain string of characters appear as listed in the text file named blackpath.txt for example bad word,deny word etc

http_access deny blacklisthttp_access deny blackpath

/etc/squid/blacklist.txt To block whole URL

http://denysite.com http://badsite.com/badcontents/

/etc/squid/blackpath.txt To block matching URL-denyword-badword

Restrict the access during particular duration only

Acl clients src 192.168.0.0/255.255.255.0Acl regular_days time MTW 10:00-12:00http_access allow clients regular_dayshttp_access deny clients

Restricting the internet usage to particular users through proxy server:-

For this purpose you have to first create users (called ncsa users whom you want to allow access to internet) using following steps :-

Create an empty file with the name squid_pass in directory /etc/squidCreate ncsa users using command# htpasswd/etc/squid/squid_pass usernameThis will asks the password for the user , give the same as asked,this creates ncsa users.

After creating ncsa users edit the /etc/squid/squid.conf as follows

Locate the line :

# auth_program /usr/bin/ncsa_auth /etc/user/passwdAnd change it as below

Auth_program /usr/lib/squid/ncsa_auth /etc/squid/squid_pass

And also insert following line under ACCESS CONTROLS of the file:

Acl ncsa_users proxy_auth REQUIRED

http_access allow ncsa_users

How to get squid started# service squid start# service squid stop or restart

Configuring web browsers to use your squid server

Internet explorer click on the “tools” option on menu bar of browserClick on internet options and clicks connections & click on LAN settings & configure with the address and TCP port (3128 default) used by your squid server

For mozilla /NetscapeClick on edit item on menu bar of browser & click on preferences & click on advanced & click on proxies & configure with IP address of your proxy server and TCP port (3128 default) used by your squid server under manual proxy configuration

Domain Name Server’s entry in /etc/squid/squid.conf file:

Locate the line dns_nameservers in squid.conf fileRemove the comment from above line and enter IP address of name servers in your network in this line as below

Dns_nameservers 202.54.6.50 203.197.12.30

This will enable the proxy server to forward the name resolution queries to these name servers for the sites indicated in the URL of browsers of clients.With this entry there is no need to give entries of ip address of name servers in /etc/resolv.conf files of client’s machines using internet through proxy server.

Lecture no #26

Reading: - G23.2-23.3, G23.6, B319-341Contents:-TCP/IP Troubleshooting: ping, traceroute, ifconfig, netstat, ipconfig

TCP/IP Troubleshooting: ping, traceroute, ifconfig, netstat, ipconfig

1 PING

Verifies IP-level connectivity to another TCP/IP computer by sending Internet Control Message Protocol (ICMP) Echo Request messages. The receipt of corresponding Echo Reply messages are displayed, along with round-trip times. Ping is the primary TCP/IP command used to troubleshoot connectivity, reachability, and name resolution. Used without parameters, ping displays help. It’s Stands for “Packet Internet Groper”

ping [-t] [-a] [-n Count] [-l Size] [-f] [-i TTL] [-v TOS] [-r Count] [-s Count] [{-j HostList | -k HostList}] [-w Timeout] [TargetName]

C:\>ping 192.168.1.110Pinging 192.168.1.110 with 32 bytes of data:

Reply from 192.168.1.110: bytes=32 time<1ms TTL=128Reply from 192.168.1.110: bytes=32 time<1ms TTL=128Reply from 192.168.1.110: bytes=32 time<1ms TTL=128Reply from 192.168.1.110: bytes=32 time<1ms TTL=128

Ping statistics for 192.168.1.110: Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),Approximate round trip times in milli-seconds: Minimum = 0ms, Maximum = 0ms, Average = 0ms

2. TRACERT

Determines the path taken to a destination by sending Internet Control Message Protocol (ICMP) Echo Request messages to the destination with incrementally increasing Time to Live (TTL) field values. The path displayed is the list of near-side router interfaces of the routers in the path between a source host and a destination. The near-side interface is the interface of the router that is closest to the sending host in the path. Used without parameters, tracert displays help.

tracert [-d] [-h MaximumHops] [-j HostList] [-w Timeout] [TargetName]

C:\>tracert 192.168.1.110Tracing route to 192.168.1.110 over a maximum of 30 hops1 <1 ms <1 ms <1 ms 192.168.1.110Trace complete.3. IPCONFIG

Displays all current TCP/IP network configuration values and refreshes Dynamic Host Configuration Protocol (DHCP) and Domain Name System (DNS) settings. Used without parameters, ipconfig displays the IP address, subnet mask, and default gateway for all adapters.

ipconfig [/all] [/renew [Adapter]] [/release [Adapter]] [/flushdns] [/displaydns] [/registerdns] [/showclassid Adapter] [/setclassid Adapter [ClassID]]

C:\>ipconfigWindows IP ConfigurationEthernet adapter Local Area Connection: Connection-specific DNS Suffix . : IP Address. . . . . . . . . . . . : 192.168.1.113 Subnet Mask . . . . . . . . . . . : 255.255.255.0 Default Gateway . . . . . . . . . : 192.168.1.254

C:\>ipconfig /allWindows IP Configuration Host Name . . . . . . . . . . . . : lab1com20 Primary Dns Suffix . . . . . . . : Node Type . . . . . . . . . . . . : Unknown IP Routing Enabled. . . . . . . . : No WINS Proxy Enabled. . . . . . . . : NoEthernet adapter Local Area Connection: Connection-specific DNS Suffix . : Description . . . . . . . . . . . : Realtek RTL8139 Family PCI Fast Ethernet NIC Physical Address. . . . . . . . . : 00-11-09-16-6B-73 Dhcp Enabled. . . . . . . . . . . : No IP Address. . . . . . . . . . . . : 192.168.1.113 Subnet Mask . . . . . . . . . . . : 255.255.255.0 Default Gateway . . . . . . . . . : 192.168.1.254 DNS Servers . . . . . . . . . . . : 192.168.1.254

IPCONFIG /RELEASE or /RENEW - Release or renew an IP Address from a DHCP Server

4. PATHPING

Provides information about network latency and network loss at intermediate hops between a source and destination. Pathping sends multiple Echo Request messages to each router between a source and destination over a period of time and then computes results based on the packets returned from each router. Because pathping displays the degree of packet loss at any given router or link, you can determine which routers or subnets might be having network problems. Pathping performs the equivalent of the tracert command by identifying which routers are on the path. It then sends pings periodically to all of the routers over a specified time period and computes statistics based on the number returned from each. Used without parameters, pathping displays help.

pathping [-n] [-h MaximumHops] [-g HostList] [-p Period] [-q NumQueries [-w Timeout] [-T] [-R] [TargetName]

C:\>pathping 192.168.1.110

Tracing route to 192.168.1.110 over a maximum of 30 hops0 lab1com20 [192.168.1.113]1 192.168.1.110Computing statistics for 25 seconds... Source to Here This Node/LinkHop RTT Lost/Sent = Pct Lost/Sent = Pct Address 0 lab1com20 [192.168.1.113] 0/ 100 = 0% | 1 0ms 0/ 100 = 0% 0/ 100 = 0% 192.168.1.110Trace complete.

5. NET

You can use the net user command to create and modify user accounts on computers. When you use this command without command-line switches, the user accounts for the computer are listed. The user account information is stored in the user accounts database. This command works only on servers.

C:\>NET HELPThe syntax of this command is:NET HELPcommand -or-NET command /HELP Commands available are: NET ACCOUNTS NET HELP NET SHARE NET COMPUTER NET HELPMSG NET START NET CONFIG NET LOCALGROUP NET STATISTICS NET CONFIG SERVER NET NAME NET STOP NET CONFIG WORKSTATION NET PAUSE NET TIME NET CONTINUE NET PRINT NET USE NET FILE NET SEND NET USER NET GROUP NET SESSION NET VIEW NET HELP SERVICES lists some of the services you can start. NET HELP SYNTAX explains how to read NET HELP syntax lines. NET HELP command | MORE displays Help one screen at a time.

C:\>NET SEND 192.168.1.104 hi!The message was successfully sent to 192.168.1.104.

C:\>NET ACCOUNTSForce user logoff how long after time expires?: NeverMinimum password age (days): 0Maximum password age (days): 42Minimum password length: 0Length of password history maintained: NoneLockout threshold: NeverLockout duration (minutes): 30

Lockout observation window (minutes): 30Computer role: WORKSTATIONThe command completed successfully.

C:\>NET CONFIGThe following running services can be controlled: Server WorkstationThe command completed successfully.

C:\>NET STATISTICSStatistics are available for the following running services: Server WorkstationThe command completed successfully.

C:\>NET USENew connections will be remembered.There are no entries in the list.

C:\>NET USERUser accounts for \\LAB1COM20---------------------------------------------------------------------------Admin Administrator GuestHelpAssistant Rajat SUPPORT_388945a0userThe command completed successfully.C:\>NET VIEWServer Name Remark---------------------------------------------------------------------------\\LAB1COM10\\LAB1COM11\\LAB1COM12\\LAB1COM13\\LAB1COM14-------------The command completed successfully.

6. NETSAT

Displays active TCP connections, ports on which the computer is listening, Ethernet statistics, the IP routing table, IPv4 statistics (for the IP, ICMP, TCP, and UDP protocols), and IPv6 statistics (for the IPv6, ICMPv6, TCP over IPv6, and UDP over IPv6 protocols). Used without parameters, netstat displays active TCP connections.

netstat [-a] [-e] [-n] [-o] [-p Protocol] [-r] [-s] [Interval]

C:\>NETSTAT -aActive Connections

Proto Local Address Foreign Address State TCP lab1com20:epmap lab1com20:0 LISTENING TCP lab1com20:microsoft-ds lab1com20:0 LISTENING

TCP lab1com20:1025 lab1com20:0 LISTENING TCP lab1com20:5000 lab1com20:0 LISTENING TCP lab1com20:netbios-ssn lab1com20:0 LISTENING UDP lab1com20:epmap *:* UDP lab1com20:microsoft-ds *:* UDP lab1com20:isakmp *:*

C:\>NETSTAT -eInterface Statistics

Received SentBytes 1283397 315664Unicast packets 2596 2617Non-unicast packets 5408 136Discards 0 0Errors 0 0Unknown protocols 36

C:\>NETSTAT -RNRoute Table===========================================================================Interface List0x1 ........................... MS TCP Loopback interface0x2 ...00 11 09 16 6b 73 ...... Realtek RTL8139 Family PCI Fast Ethernet NIC - Packet Scheduler Miniport======================================================================================================================================================Active Routes:Network Destination Netmask Gateway Interface Metric 0.0.0.0 0.0.0.0 192.168.1.254 192.168.1.113 20 127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1 192.168.1.0 255.255.255.0 192.168.1.113 192.168.1.113 20 192.168.1.113 255.255.255.255 127.0.0.1 127.0.0.1 20 192.168.1.255 255.255.255.255 192.168.1.113 192.168.1.113 20 224.0.0.0 240.0.0.0 192.168.1.113 192.168.1.113 20 255.255.255.255 255.255.255.255 192.168.1.113 192.168.1.113 1

Default Gateway: 192.168.1.254===========================================================================Persistent Routes: None

C:\>NETSTAT -OActive Connections Proto Local Address Foreign Address State PID

C:\>NETSTAT -NActive Connections Proto Local Address Foreign Address State

C:\>NETSTAT -P TCPActive Connections Proto Local Address Foreign Address State

C:\>NETSTAT -RRoute Table===========================================================================Interface List0x1 ........................... MS TCP Loopback interface0x2 ...00 11 09 16 6b 73 ...... Realtek RTL8139 Family PCI Fast Ethernet NIC - Packet Scheduler Miniport======================================================================================================================================================Active Routes:Network Destination Netmask Gateway Interface Metric 0.0.0.0 0.0.0.0 192.168.1.254 192.168.1.113 20 127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1 192.168.1.0 255.255.255.0 192.168.1.113 192.168.1.113 20 192.168.1.113 255.255.255.255 127.0.0.1 127.0.0.1 20 Default Gateway: 192.168.1.254===========================================================================Persistent Routes:None

C:\>NETSTAT -SIPv4 Statistics Packets Received = 6912 Received Header Errors = 0 Received Address Errors = 123 Datagrams Forwarded = 0 Unknown Protocols Received = 0 Received Packets Discarded = 0 Received Packets Delivered = 6873 Output Requests = 2727

7 IFCONFIG

ifconfig checks the network interface configuration. Use this command to verify the user's configuration if the user's system has been recently configured or if the user's system cannot reach the remote host while other systems on the same network can.

When ifconfig is entered with an interface name and no other arguments, it displays the current values assigned to that interface. For example, checking interface dnet0 on a Solaris 8 system gives this report:

% ifconfig dnet0

dnet0: flags=1000843<UP,BROADCAST,RUNNING,MULTICAST,IPv4> mtu 1500 index 2

inet 172.16.55.105 netmask ffffff00 broadcast 172.16.55.255

The ifconfig command displays two lines of output. The first line of the display shows the interface's name and its characteristics. Check for these characteristics:

UP

The interface is enabled for use. If the interface is "down," have the system's superuser bring the interface "up" with the ifconfig command (e.g., ifconfig dnet0 up). If the interface won't come up, replace the interface cable and try again. If it still fails, have the interface hardware checked.

RUNNING

This interface is operational. If the interface is not "running," the driver for this interface may not be properly installed. The system administrator should review all of the steps necessary to install this interface, looking for errors or missed steps.

The second line of ifconfig output shows the IP address, the subnet mask (written in hexadecimal), and the broadcast address. Check these three fields to make sure the network interface is properly configured.

Two common interface configuration problems are misconfigured subnet masks and incorrect IP addresses. A bad subnet mask is indicated when the host can reach other hosts on its local subnet and remote hosts on distant networks, but it cannot reach hosts on other local subnets. ifconfig quickly reveals if a bad subnet mask is set.

An incorrectly set IP address can be a subtle problem. If the network part of the address is incorrect, every ping will fail with the "no answer" error. In this case, using ifconfig will reveal the incorrect address. However, if the host part of the address is wrong, the problem can be more difficult to detect. A small system, such as a PC that only connects out to other systems and never accepts incoming connections, can run for a long time with the wrong address without its user noticing the problem. Additionally, the system that suffers the ill effects may not be the one that is misconfigured. It is possible for someone to accidentally use your IP address on his system, and for his mistake to cause your system intermittent communications problems. An example of this problem is discussed later. This type of configuration error cannot be discovered by ifconfig because the error is on a remote host. The arp command is used for this type of problem.

8 TRACEROUTE

If the local routing table is correct, the problem may be occurring some distance away from the local host. Remote routing problems can cause the "no answer" error message, as well as the "network unreachable" error message. But the "network unreachable" message does not always signify a routing problem. It can mean that the remote network cannot be reached because something is down between the local host and the remote destination. traceroute is the program that can help you locate these problems.

Traceroute traces the route of UDP packets from the local host to a remote host. It prints the name (if it can be determined) and IP address of each gateway along the route to the remote host.

Traceroute uses two techniques, small TTL (time-to-live) values and an invalid port number, to trace packets to their destination. traceroute sends out UDP packets with small TTL values to detect the intermediate gateways. The TTL values start at 1 and increase in increments of 1 for each group of three UDP packets sent. When a gateway receives a packet, it decrements the TTL. If the TTL is then 0, the packet is not forwarded and an ICMP "Time Exceeded" message is returned to the source of the packet. traceroute displays one line of output for each gateway from which it receives a "Time Exceeded" message. Figure 13-2 presents a sample of the single line of output that is displayed for a gateway, and shows the meaning of each field in the line.

When the destination host receives a packet from traceroute, it returns an ICMP "Unreachable Port" message. This happens because traceroute intentionally uses an invalid port number (33434) to force this error. When traceroute receives the "Unreachable Port" message, it knows that it has reached the destination host, and it terminates the trace. So, traceroute is able to develop a list of the gateways, starting at one hop away and increasing one hop at a time until the remote host is reached. Figure 13-3 illustrates the flow of packets tracing to a host three hops away. The following shows a traceroute to www.internic.net from a Solaris system hanging off the Comcast network. traceroute sends out three packets at each TTL value. If no response is received to a packet, traceroute prints an asterisk (*). If a response is received, traceroute displays the name and address of the gateway that responded and the packet's round trip time in milliseconds

$ traceroute www.internic.net

traceroute to www.internic.net (207.151.159.3), 30 hops max, 40 byte packets

1 ani (192.168.0.1) 1.712 ms 1.40 ms 1.34 ms

2 10.81.130.1 (10.81.130.1) 52.01 ms 34.38 ms 118.97 ms

3 bb1-fe1-0.mtgmry1.md.home.net (24.11.248.1) 13.30 ms 100.92 ms 31.99 ms

4 c2-se9-0-10.washdc1.home.net (24.7.73.25) 118.63 ms 94.92 ms 121.10 ms

5 24.7.71.6 (24.7.71.6) 127.63 ms 26.29 ms 132.07 ms

6 p4-6-1-0.r00.plalca01.us.bb.verio.net (129.250.2.245) 186.02 ms 164.81 ms 156.44 msSo on -------------------

Configuring a Linux/Windows Box as a Router, Dialup Configuration and Authentication: PPP Radius, RAS