ca_ex_s2m01_introduction to routing and packet forwarding

8/3/2019 CA_Ex_S2M01_Introduction to Routing and Packet Forwarding

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Chapter 1 - Introduction to Routing and

Packet Forwarding

CCNA Ex loration 4.0

Please purchase apersonal license.


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Objectives

Identify a router as a computer with an OS and hardwaredesigned for the routing process.

Demonstrate the ability to configure devices and applyaddresses. Describe the structure of a routing table.

Hc vin mng Cisco Bch Khoa - Website: www.ciscobachkhoa.com 2

packets


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Labs



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Inside the Router



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Routers are Computers

Routers have many of the same hardware and software componentsthat are found in other computers including:

CPU

RAM

ROM

Operating System



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Routers are at the network center A router connects multiple networks. This means that it has multiple interfaces

that each belong to a different IP network.

Each network that a router connects to typically requires a separate interface.These interfaces are used to connect a combination of both Local Area

Networks (LANs) and Wide Area Networks (WANs).



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Routers determine the best path

The primary responsibility of a router is to direct packets destined for local andremote networks by:

Determining the best path to send packets

Forwarding packets toward their destination


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Router as a Computer

Describe the basic purpose of a routerComputers that specialize in sending packets over

the data network. They are responsible forinterconnecting networks by selecting the best pathfor a packet to travel and forwarding packets totheir destination


Routers are the network centerRouters generally have 2 connections:

WAN connection (Connection to ISP)

LAN connectionNot common:

PSTN connection


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Router as a Computer


"How Routers Work"http://computer.howstuffworks.com/router.htm

Activity 1.1.1.4


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Evolution of Routers

Multiprotocol devices providing pure data delivery with


.

Intelligent Network Services, this allowed us to providesecurity capabilities like integrated firewall and encryption.

Integrated voice and data capabilities, allowing routers to

provide voice gateway features for IP telephony. Today the router is an integrated platform for providing

advanced services in security, content delivery, and voice.


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Cisco Router Series



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Extra: Integrated Services Routers


The network router is quickly evolving from a device dedicated to connecting disparatenetworks to an integrated services device capable of multiple functions beyond routing.

Now, Integrated Services Routers can deliver voice, video, data and Internet access,wireless, and other applications.

Benefits of the Integrated Services Network Router

One Device, Multiple Functions Same Access at Headquarters and Remote Sites

Centralized Management

Integrated Network Security


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Router Components

Router components and their functions CPU - Executes operating system instructions

Random access memory (RAM) - Contains the running copy of

configuration file. Stores routing table. RAM contents lost whenpower is off

Read-only memory (ROM) - Holds diagnostic software used whenrouter is powered up. Stores the routers bootstrap program.


on-vo a e - ores s ar up con gura on. s

may include IP addresses (Routing protocol, Hostname of router) Flash memory - Contains the operating system (Cisco IOS)

Interfaces - There exist multiple physical interfaces that are used toconnect network. Examples of interface types:

Ethernet / FastEthernet interfaces Serial interfaces

Management interfaces


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Router Components



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Router Components



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Cisco router 1841s Architecture - Front Panel


LED Color Status

SYS PWR Green Router has successfully booted up and the software is functional. Slow,

steady blinking when system is booting or in the ROM monitor.

SYS ACT Green Blinking when packets are transmitted or received on any WAN or LAN

interface, or when monitoring system activity.

CF Blinking

green

Flash memory is busy. Do not remove the CompactFlash memory card

when this light is on.

http://www.cisco.com/cdc_content_elements/flash/nextgen/webversion/1800/kaon/1841/index.html?NO_NAV


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Cisco router 1841s Architecture - Back Panel



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Cisco router 1841s Architecture - Internal


A SO-DIMM, or small outline dual in-line memory module, is a type ofcomputer memory integrated circuit


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Demo: View Interactive 3D Model of the Cisco1841 Integrated Services Router

http://www.cisco.com/cdc_content_elements/flash/nextgen/webversion/1800/kaon/1841/index.html?NO_NAV



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Router Components

Activity 1.1.2.2



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Internetwork Operating System


Cisco IOS manages the hardware and software resources of the router,including memory allocation, processes, security, and file systems. Cisco IOS

is a multitasking operating system that is integrated with routing, switching,internetworking, and telecommunications functions.

Cisco creates many different types of IOS images, depending upon the modelof the router and the features within the IOS.

The command line interface (CLI) is a much more common method of

configuring Cisco routers.


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Cisco IOS Software Architecture for 1800 Series

Identified IOS Software includes: Feature and Version



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Cisco IOS Packaging


Baseentry level image (IP Base, Enterprise Base) Servicesaddition of IP Telephony Service, MPLS, Voice over IP

(VoIP), Voice over Frame Relay (VoFR), and ATM (SP Services,Enterprise Services)

Advancedaddition of VPN, Cisco IOS Firewall, 3DES encryption,SSH, Cisco IOS IPsec and Intrusion Detection Systems (IDS)(Advanced Security, Advanced IP Services)

Enterpriseaddition of multi-protocols, including IBM, IPX, AppleTalk(Enterprise Base, Enterprise Services)


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Product Research Tools



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Router Boot-up Process

1. Performing the POST2. Loading the bootstrap program3. Locating and loading the Cisco IOS software

4. Locating and loading the startup configuration file or entering setupmode



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1. ROM

1. POST

2. Bootstrap code executed

3. Check Configuration Register value (NVRAM)

Forth Hex : 0 = ROM Monitor mode

1 = ROM IOS2 - 15 = startup-config in NVRAM

2. Check for IOS boot system commands in startup-config file (NVRAM)

If boot system commands in startup-config

Extra: Router Boot Process

1, 2

3

4


a. Run boot system commands in order they appear in startup-config to locate the IOS

b If boot system commands fail, use default fallback sequence to locate the IOS (Flash, TFTP, ROM)

3. Locate and load IOS, Default fallback sequence: No IOS boot system commands in startup-config

a. Flash (sequential)

b. TFTP server (netboot) - The router uses the configuration register value to form a filename fromwhich to boot a default system image stored on a network server.

c. ROM (partial IOS) or keep retrying TFTP depending upon router model- If no IOS located, get partial IOS version from ROM

4. Locate and load startup-config

a. If startup-config found, copy to running-config

b. If startup-config not found, prompt for setup-mode

c. If setup-mode bypassed, create a skeleton default running-config (no startup-config)


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Extra: Router Boot Process



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Extra: Check for IOS boot system commands in startup-config file (NVRAM)



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R


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Router Interfaces


R d h N k L


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Routers and the Network Layer

The main purpose of a router is to connect multiple networks and forwardpackets destined either for its own networks or other networks.

A router is considered a Layer 3 device because its primary forwardingdecision is based on the information in the Layer 3 IP packet, specifically the

destination IP address. This process is known as routing.


R t d th N t k L


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Routers and the Network Layer

Routers Operate at Layers 1, 2, and 3



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CLI Configuration and Addressing


Implementing Basic Addressing Schemes


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When designing a new network or mapping an existingnetwork, document the network. At a minimum, thedocumentation should include a topology diagram that

indicates the physical connectivity and an addressing tablethat lists all of the following information:

Device names


Interfaces used in the design IP addresses and subnet masks

Default gateway addresses for end devices, such asPCs



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Activity 1.2.1.1




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Basic Router Configuration


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Router name

Passwords

Banner




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Router Interface Configuration




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Activity 1.2.2.3 Verifying Basic Router Configuration

Issue the show running-config command

Save the basic router configuration by Issuing the

copy running-config startup-config command


configuration are: show running-config - Displays configuration currently in RAM

show startup-config - Displays configuration file NVRAM

show ip route - Displays routing table

show interfaces - Displays all interface configurations

show ip interface brief - Displays abbreviated interfaceconfiguration information


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Building the Routing Table


Introducing the Routing Table


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The primary function of a router is to forward a packettoward its destination network, which is the destination IPaddress of the packet.

Routing Table is stored in RAM and contains informationabout:

-


is connected to another router interface Remotely connected networks - this is a network that is

not directly connected to a particular router

Detailed information about the networks include sourceof routing information, Network address & subnet mask,and ip address of next-hop router, exit interface

show ip route command is used to view a routing table



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Directly Connected Routes Static Routes Dynamic Routes


Directly-Connected Networks


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Directly Connected Networks

Once the interface is "up," the network of that interface isadded to the routing table as a directly connected network.


Static Routing


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Static Routing

Static routes in the routing table Includes: network address and subnet mask and IP

address of next hop router or exit interface

Denoted with the code S in the routing table

Routing tables must contain directly connected


or dynamic routing can be used When to use static routes

When network only consists of a few routers

Network is connected to internet only through one ISPHub-and-spoke topology is used on a large network

Static Routing


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Static Routing


Dynamic Routing


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Dynamic routing protocols are used by routers to share informationabout the reachability and status of remote networks. Dynamic routingprotocols perform several activities, including:

Network discovery Updating and maintaining

routing tables


Dynamic Routing


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Dynamic routing features: Automatic Network Discovery: allows the routers to automatically

learn about these networks from other routers.

Maintaining Routing Tables: automatically share routinginformation with other routers and compensate for any topologychanges without involving the network administrator.


RIP (Routing Information Protocol) IGRP (Interior Gateway Routing Protocol)

EIGRP (Enhanced Interior Gateway Routing Protocol)

OSPF (Open Shortest Path First)

IS-IS (Intermediate System-to-Intermediate System) BGP (Border Gateway Protocol)

Routing Table Principles


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Activity 1.3.5.1 3 principles regarding routing tables:

1. Every router makes its decisions alone, based on the information ithas in its routing table.

2. One routers information in the routing table may not be the sameas another routers routing table.

3. Routing information about a path to a network does not provide


n orma on on ow o re urn o e or g na ne wor .



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Path Determinationand Switchin Functions


Packet Fields and Frame Fields


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Internet Protocol (IP) packet format contains fields that provide informationabout the packet and the sending and receiving hosts

Fields that are importance for CCNA students: Destination IP address

Source IP address

Version & TTL

IP header length


Precedence & type of service

Packet length

Packet Fields and Frame Fields


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MAC Layer Frame Format MAC Frames are also divided into fields. They include:

Preamble

Start of frame delimiter

Destination MAC address

Source MAC address

Type/length


Data and pad

Frame check sequence

Best Path and Metric


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A Metric is a numerical value used by routing protocols to assess agiven route

Metrics may be based on a single path variable or multiple variables ofa path

2 types of metrics used by routing protocols are:

1. Hop count - this is the


packet must travelthrough to get to itsdestination

2. Bandwidth - this is thespeed of a link also

known as the datacapacity of a link

Router Paths and Packet Switching


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Equal cost metric is a condition where a router has multiple paths tothe same destination that all have the same metric

To solve this dilemma, a router will use Equal Cost Load Balancing.This means the router sends packets over the multiple exit

interfaces listed in the routing table. Equal Cost Paths and Unequal Cost Paths: a router can sendpackets over multiple networks even when the metric is not thesame if it is using a routing protocol that has this capability.


Path Determination


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The path determination function is the process of how the routerdetermines which path to use when forwarding a packet by

searching its routing table.

One of three path determinations results from this search: Directly connected network

Remote network


No route determined

Switching Function


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Switching Function of Router is the process used by arouter to switch a packet from an incoming interface to anoutgoing interface on the same router.

A packet received by a router will do the following: Strips off layer 2 headers.


header to find best route to destination. Re-encapsulates layer 3 packet into layer 2 frame.

Forwards frame out exit interface.

Switching Function


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Step 1: PC1 has a packet to be sent to PC2


Switching Function


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Switching Function


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Switching Function


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Step 2: Router R1 receives the Ethernet frame


Switching Function


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Switching Function


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Switching Function


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Switching Function


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Switching Function


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Switching Function


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Step 3: Packet arrives at router R2


Switching Function


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Switching Function


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Switching Function


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Switching Function


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Switching Function


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Switching Function


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Switching Function


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Step 4: The packet arrives at R3


Switching Function


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Switching Function


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Switching Function

S 4 Th k i R3


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Switching Function

St 4 Th k t i t R3


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Switching Function

St 4 Th k t i t R3


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Step 5:The Ethernet Frame with encapsulated IP packet

arrives at PC2

Switching Function

Step 5: The Ethernet Frame with encapsulated IP packet


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Step 5: The Ethernet Frame with encapsulated IP packetarrives at PC2

1. PC2 examines the destination MAC address, which

matches the MAC address of the receiving interface,its Ethernet NIC. PC2 will therefore copy the rest of theframe into its buffer.


2. PC2 sees that the Ethernet Type field is 0x800, whichmeans that the Ethernet frame contains an IP packetin the data portion of the frame.

3. PC2 decapsulates the Ethernet frame and passes the

IP packet to the IP process of its operating system.

Summary


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Configuration Register Setting 1800 Series

The order in which the router looks for system bootstrap information


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The order in which the router looks for system bootstrap informationdepends on the boot field setting in the configuration register.

The configuration register is a 16-bit register in NVRAM. To ensure that the upper 12 bits are not changed, first retrieve the

current values of the configuration register using the show versioncommand.

-


the last hexadecimal digit.

Configuration Register


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Boot Field Configuration Register Bit Descriptions

Boot Field

(Bits 3, 2,

1, and 0)

Meaning

0000

(0x0)

At the next power cycle or reload, the router boots to the ROM monitor.


(0x01)

oo s e rs mage n as memory as a sys em mage.

0010 - 1111

(0x02 - 0xF)

At the next power cycle or reload, the router sequentially processes each

boot system command in global configuration mode that is stored in the

configuration file until the system boots successfully.

If no boot system commands are stored in the configuration file, or if

executing those commands is unsuccessful, then the router attempts to

boot the first image file in flash memory.



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ca_ex_s2m01_introduction to routing and packet forwarding

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