wide area networking

WIDE AREA NETWORKINGYou typically use straight through Ethernet cables (PINS 1, 2, 3, 6) between end user devices and the

switches. For the trunk links between the switches, you use crossover cables because each switch transmits on the same pair, so the crossover cable connect one device’s receive pair. The lower part of the figure reminds you of the basic idea behind a crossover cable.

The term clock rate and band width both refer to the speed of the circuit. You will also hear the speed referred to as the link speed.

Clock Rate and Band width both mean SPEED of the Circuit

When you order a circuit that runs at a particular speed, the two CSU/DSU are configured to operate at the same speed. The CSU/DSU’s is providing a clocking signal to the routers so that the router simply reacts, sending and receiving data at the correct rate. So, the CSU/DSU is considered to be clocking the link.

Typically, routers are connect to a device called and external

Channel Service Unit/ Data Service Unit (CSU/DSU)

CPE – Customer premises equipment – refers to devices that are at the customer site.

The device that provides clocking typically the CSU, is considered to be the

Data Communications Equipment (DCE)

The device receiving clocking, typically the router, is referred to as:

Data Terminal Equipment (DTE)

When purchasing serial cables form Cisco, you can pick either a DTE or a DCE Cable. You pick the type of cable based on whether the router is acting like a DTE or a DCE

If the router is a DTE, with the CSU providing the clocking you need a DTE cable.

If the router was clocking the CSU/DSU, which can be done, you would need a DCE cable but that almost never happens.

Synchronization- Circuits impose time ordering at the link sending and receiving ends. Essentially all devices agree to try to run at the exact same speed.

You can buy two routers, a DTE serial cable for one router, and a DCE serial cable for the other and connect the two cables together. The router with the DCE cable in it can be configured to provide clocking. Meaning that you do not need to have a CSU/DSU in-between because clocking is being generated by the link in the test.

The DTE and the DCE cables can be connected to each other and the two routers.

The DCE cable has a female connector

The DTE has a male connector, so they can be connected together.

With one additional configuration command on the routers:

The #clock rate command, will give you a point to point serial link. This type of connection between tow routers sometimes is called a back-to-back serial connection.

For any of the point-to-point serial link or Frame Relay links the router uses an interface that supports Synchronous communication.

The cable between the CSU/DSU and the Telco CO typically uses an RJ-48 connector to connect to the CSU/DSU; the RJ-48 connector has the same size and shape as the RJ-45 connector used for Ethernet cables.

The original standard for converting analog voice to a digital signal is called Pulse Code Modulation (PCM). PCM defines that an incoming analog voice signal should be sampled 8000 times per second, and each sample should be represented by an 8-bit code. So, 64, 000 bits were needed to represent 1 second of voice.

Voice = 60 * 8000 = 64,000 or 64K

Point to point WANS are made up to two routers connected by CSU/DSU:

Router ------- CSU ----CO---- CSU -------- Router

Frame Relay WANS are made up of Multi-access network, which means that more then two devices can attach to the network similar to LAN:

Router--Access Link-----DCE (Frame Relay Switch) -- DCE –Access Link-Router

Frame Relay does not use CSU/DSU only Point to Points uses them.

There are two types of Packet switching ====Frame Relay and ATM,

Components of Point to Point leased lines

Back to back serial cabling

Clock rate command is used on the DCE router.

Command

Router (config) # serial 0/0

Router (config-int) # clockrate 56000

CSU/DSU are only used on point to point serial lines.

Data Link Protocols for Point to Point Leased Lines

HDLC – High Level Data Link Control are Wan Data-Link Protocols. Only used by cisco proprietary.

HDLC & PPP are used on point serial links.

PPP can also use Asynchronous serial links.

Data Links use the concept of Framing.

WAN data links are Frame-oriented.

Frame data which is encased in between a header and trailer.

Synchronous WAN links require the CSU/DSU on each side of the WAN link to operate at the same speed.

Asynchronous the speed does not have to be the same.

FCS is used to perform error detection all WAN Data Link protocols perform error detection HDLC & PPP.

FCS is located at the end of the frame and is 4 bytes long.

HDLC needs a CSU/DSU because it only runs Synchronously.

Command

Router# encapsulation HDLC or PPP

<HDLC is default>

To switch back to HDLC from PPP use

Router # no encapsulation PPP

Or

Router # encapsulation HDLC

Show Commands used to display information on Point to Point Interfaces:

Router # Show interfaces

Router # Show compress

Router # Show process

Point-to-Point Circuits use a CSU-DSU.

CSU- Channel Service Unit

DSU – Data Service Unit

Example of an Pont to Pont Hardware Configuration:

Router A Equipment Equipment Router B

Slave Master

DCE WAN Switches at CO DCE

Customer Telephone Co Customer

DTE Equipment Equipment Equipment DTE

Equipment Equipmnet

Point to Point Circuits

CSU/DSU TELCO SWITCH CSU/DSU

CSU-DSU – are configured to operate at the same Speed

CSU/DSU – Provide the Clocking Signals to the Routers so that the routers simply react sending and receiving data at the correct rate.

CSU-DSU – are considered to be the Clocking link.

Point to Point protocols

HDLC- (Default type in all Point to Point Configurations) – Only uses Synchronous

Configuration Value.

PPP- uses CHAP – Can use Asynchronous and Synchronous configurations speeds.

SDLC

LAPB

LAPD – Used with ISDN

LAPF – Used only for Frame Relay

Q922- (Data Link Protocols) Encapsulation

Q933- Handles the Establishment and Tear-Down of the circuit.

FRAME Relay Configuration

Does not use/Need CSU DSU

Telephone Company (CO)

Frame Switching

Router A Equipment Router B

DTE DCE DCE DTE

Circuit CLOCKING Equipment

The Frame Switch Provides the CLOCKING for the Routers (DTE) Equipment so no External CSU/DSU is needed for the Circuit.

Protocol used between the Router (DTE) and the Frame Switch (DCE).

LMI -Local Management Interface – Sends Keep Alive messages between the DTE (Router) and the DCE (Frame Switch) circuit equipment.

LMI ( Local Management Interface)

LMI is used between the DTE and DCE to send messages about the stats of the circuit there main duties are to send:

Keep alive messages

Active or Inactive messages

Both DCE and DTE must use the same LMI

DCE – Data communication Equipment (Frame Switch)

DTE – Data Terminal Equipment (Router)

LMI – Local Management Interface protocol

You can use LMI auto sense feature so the router can automatically configure LMI type.

Frame Switch

Frame Switch

LMI types

TYPE IOS command/Display

Cisco Default/Proprietary Cisco

ANSI t1.617 ANSI

ITU q.933 q.933a

LMI- is a definition of the messages used between the DTE and the DCE equipment.

By Default the following information is automatically discovered or configured:

The LMI is automatically Sensed

The encapsulation is Cisco by default the other choice is IETF

PVC (Layer 2 Data-Link) DLCI are Leaned via LMI status messages

Inverse ARP is enabled (By default) and is triggered when the status messages declaring that the VC’s are up is received. Inverse ARP finds a Layer 3 (Network (IP protocol)) protocol to a Layer 2 (Data Link (DLCI)) protocol.

You must use IETF Encapsulation if one Router is not a Cisco Router.

Frame Relay Commands used to configure a Basic Frame Relay interface:

Command:

Router# Configure Terminal

Router (config) # Interface Serial 0

Router (config) # IP address 199.1.1.1 255.255.255.0

Router (config-if) #Encapsulation Frame-Relay

Router (config-if) # Frame-Relay Type ANSI Other choices (CISCO, q933a (ITU))

Router (config-if) #Frame-Relay interface-DLCI 53 CISCO (Default) Other choice is IETF

Since the Default configuration Automatically use the default configuration the only thing needed to configure on a Full mesh Network or one IP subnet Network for its Subinterface’s is:

Command:

Router# Configure Terminal


Router (config-if) # Encapsulation Frame Relay

Router (config) # IP address 199.1.1.1 255.255.255.0

The DLCI, Frame-Relay Type are automatically configured by the Hello LMI as long as all equipment are CISCO equipment and Default Values are used as far as the LMI is concerned.

When you configure Frame Relay on the Physical interface, all Interfaces must be in the same SUBNET.

The Encapsulation Frame Relay Command tells the Router to use Frame Relay Data Link Protocols instead of the default, which is HDLC for Point to point VC.

Command:

Router (config-if) # Encapsulation Frame-Relay

Frame Relay “Mapping” Creates a correlation between a Layer 3 (NETWORK IP ADDRESS) Address and its corresponding Layer 2 (DATA-LINK DLCI) address.

Layer 3 Network IP 199.1.1.1 (32 bit’s)

Layer 2 DATA LINK LANS use: MAC ADDRESS (48 bit’s)

WANS use: DLCI (DATA Link Connection Identifier)

With IP Address Resolution Protocol (ARP) Cache used on LAN’s is an example of a Layer 3 to Layer 2 (Data Link) address Mapping.

With IP ARP, you know the IP address of another device on the same LAN, but not the MAC address; When ARP completes, you know another device LAN (Layer 2 MAC) address.

Similarly, Routers that use Frame Relay need a Mapping between a router’s Layer 3 address and the DLCI used to reach that other Router.

The information that correlates to the Next HOP Router’s Layer 3 address, and the Layer 2 address used to reach it, is called Mapping. Mapping is needed on Multi-Access Networks like Frame Relay.

To show the DLCI on a Router use the following command. This Mapping is generated by the LMI status messages.

Command:

Router # Show Frame-Relay PVC

To show the Mapping of Layer 3 to Layer 2 use the following:

Command:

Router# Show Frame Relay Map

Types of Circuits

VC – Virtual Circuit

PCV- Permanent Virtual Circuits (always on)

SVC- Switched Virtual Circuits (Turns on only when needed they are Dynamic)

Data Link Connection Identifier (DLCI)

NBMA – Non-Broadcast Multi-access- Used to send router updates on frame relay circuits.

Frame Relay networks are Multi-access Meaning that more than two devise can access/attach to the network, similar to LANs. Unlike LANs, you can-not send data link broadcasts over Frame Relay.

Frame Relay Networks are called Non-broadcast Multi-access (NBMA) Networks.

Frame Relay Switch are DCE data Communication Equipment.

DTE – Data terminal Equipment (Routers)

Router use Data Link connection identifiers (DLCI) as the Frame Relay address, which identifies the VC over which the frame should travel.

CIR- Committed Information Rate guarantees x amount of band width from the Telco.

Routers must use the same type of LMI protocol for communication.

Link Access Procedure Frame Bearer Services (LAPF)

Specification ITU q.922-a

LAPF- Framing provides error detection with a FCS in the trailer as well as the DLCI, DE, FECN and BECN

There are type field in the Frame Relay Frame But two solutions were created to solve the problem:

2byte protocol type field between the LAPF and the data packet.

RFC 1490 or updated RFC 2427 Multi-protocol Interconnect over Frame- Relay field also between the LAPF and the data packet.

DTE must agree on the encapsulation type the two types are:

CISCO IETF

DLCI is the Frame relay address

Frame Relay DLCI are Locally Significant- this means that the addresses need to be unique only on the local access link

Command

Router (config-int) # frame Relay DCLI 52

Frame Relay Address Mapping

ARP- Address Resolution Protocol- Frame Relay “mapping” creates a correlation between Layer 2 (MAC address) and Layer 3(IP address) addresses.

The information that correlates to the next-hop routers layer 3 address (IP) and the layer 2 (MAC address used to reach it, is called mapping.

Mapping is need on multi-access networks.

Command

Router# Show Frame Relay Map

&

Router# Show Frame Relay PVC

Tells you the DLCI number for the router PVC Interface.

Routers can user two methods to build the mapping:

Static Map

Dynamic process called Inverse ARP

Inver (Inverse) ARP – dynamically creates mapping between the Layer 3 (IP) address and the Layer 2 (MAC) address

The process used by Inverse ARP differs for ARP on a LAN. After the VC is up, each router announces its network layer address by sending an Inverse ARP message over the VC.

Inverse ARP stats by learning the DLCI data-link layer address (via LMI messages) and then it announces it own Layer 3 address that uses that VC.

Inverse ARP is enabled by Default on Cisco Routers

To turn off Inverse ARP Command:

Router# no frame relay inverse arp

Two Methods are used to build the Mapping for Layer 3 to Layer 2 protocols:

One way is to Statically configure it

Second Way is to let it happing Dynamically using the process called Inverse ARP.

Inverse ARP starts by Learning the DLCI data Link Layer address (VIA LMI status Message) and then it announces its own Layer 3 (Network, IP) address that uses that VC.

Inverse ARP is Enabled by default.

When using Frame Relay Subinterface’s you do not give Serial 0 an IP address you would only enable Encapsulation Frame-Relay command.

Command:

Router # Interface Serial 0

Router (config-int) # Encapsulation Frame Relay

Three Basic types of Frame Relay Network physical Configurations:

1) ONE SUBNET-------------- Full Mesh

2) Point-to-Point-------------- Partial Mesh

3) Point-to-Multi-Point------ Partially Meshed with some Fully Meshed parts

ONE SUBNET -------------- Full Mesh

Point-to-Point-------------- Partial Mesh

Three Different Subnets per Set of VC

Point-to-Multi-Point------ Partially Meshed with some Fully Meshed parts

Multiple Subnets

Fully Meshed Side Partial Meshed Side

Same Subnet Different Subnet

= This is equal to a Virtual Circuit used on Frame Relay

Full mesh Configuration for networks with Redundant links. This method uses one Sub-Network for all its Router interfaces.

Static Mapping configuration for one Subnet Configuration

Command:

Router# interface Serial 0

Router (config-int) # No Frame Relay Inverse ARP (Disables Inverse ARP

Router (config-int) # Frame-Relay Map IP 199.1.1.1 255.255.255.0 51 Broadcast

Router (config-int) # Frame-Relay Map IP 199.1.1.2 255.255.255.0 52 Broadcast

This basically means when you send information out 199.1.1.1 it will be going out DLCI 51 and to send information to 199.1.1.1 you would use DLCI 51 and the same for DLCI 52. This DLCI basically tell the router which VC to use to send the data to the next router.

Partial Mesh using Pint-To-Point Subinterface’s configuration

Command:


Router (config-if) # Encapsulation Frame Relay

Router (config-if) # Interface Serial 0.1 Point to Point

Router (config-if) # Ip address 140.1.1.1 255.255.255.0

Router (config-if) # Frame-Relay interface-DLCI 53

Router (config-fr-dlci) #Interface Serial 0.2 Pint-to-Point

Router (config-sub-if) # IP address 140.1.2.1 255.255.255.0

Router (config-sub-if) #Frame-Relay interface-DLCI 53

Notice the Point-to-Point configuration on serial 0.1 and 0.2 also notice the command prompt modes when commands are placed.

Partially-Meshed Network with some Fully-Meshed Parts configuration:

Commands:

Router (Config) # Interface Serial 0

Router (Config-if) # Encapsulation Frame-Relay

Router (config-if) # Interface Serial 0.1 Multi-Point

Router (config-if) # Ip address 140.1.1.1 255.255.255.0

Router (config-if) # Frame-Relay interface-DLCI 503

Router (config-if) # Frame-Relay Interface-DLCI 502

Router (config-fr-dlci) #Interface Serial 0.2 Pint-to-Point

Router (config-sub-if) # IP address 140.1.2.1 255.255.255.0

Router (config-sub-if) #Frame-Relay interface-DLCI 504

Multipoint Means that there is more than one VC, so you can send and receive to and form more than one VC on the Subinterface.

Only one Frame-Relay-DLCI command is allowed on Point to Point Subinterface’s, because only one VC is allowed.

Multipoint Interfaces must be in the same Subnet for them to work.

Point-to-Point –Each sent to interfaces are in its own subnet.

Configuring Frame Relay

The Cisco Frame Relay MIB adds extensions to the standard Frame Relay MIB (RFC 1315). It provides additional link-level and virtual circuit-level information and statistics that are mostly specific to Cisco Frame Relay implementation. This MIB provides SNMP network management access to most of the information covered by the show frame-relay commands, such as, show frame-relay lmi, show frame-relay pvc, show frame-relay map, and show frame-relay svc.

Frame Relay Hardware Configurations

You can create Frame Relay connections using one of the following hardware configurations:

Connect routers and access servers directly to the Frame Relay switch. Connect routers and access servers directly to a channel service unit/digital service unit

(CSU/DSU), which then connects to a remote Frame Relay switch.

Note Routers can connect to Frame Relay networks either by direct connection to a Frame Relay switch or through CSU/DSUs. However, a single router interface configured for Frame Relay can only be configured for one of these methods.

The CSU/DSU converts V.35 or RS-449 signals to the properly coded T1 transmission signal for successful reception by the Frame Relay network. Figure 9 illustrates the connections between the different components.

Figure 9 Typical Frame Relay Configuration

The Frame Relay interface actually consists of one physical connection between the network server and the switch that provides the service. This single physical connection provides direct connectivity to each device on a network.

http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/12cgcr/wan_c/wcfrelay.htm#35293

Frame Relay Configuration Task List

You must follow certain required, basic steps to enable Frame Relay for your network. In addition, you can customize Frame Relay for your particular network needs and monitor Frame Relay connections. The following sections outline these tasks.

The tasks described in the following sections are required:

Enable Frame Relay Encapsulation on an Interface Configure Dynamic or Static Address Mapping

The tasks described in the following sections are optional and are used to customize Frame Relay:

Configure the LMI Configure Frame Relay Switched Virtual Circuits Configure Frame Relay Traffic Shaping Customize Frame Relay for Your Network Monitor and Maintain the Frame Relay Connections

Enable Frame Relay Encapsulation on an Interface

To set Frame Relay encapsulation at the interface level, use the following commands beginning in global configuration mode:

Step Command Purpose

1. interface type number Specify the interface, and enter interface configuration mode.

2. encapsulation frame-relay [ietf]

Enable Frame Relay, and specify the encapsulation method.

Frame Relay supports encapsulation of all supported protocols in conformance with RFC 1490, allowing interoperability between multiple vendors. Use the Internet Engineering Task Force (IETF) form of Frame Relay encapsulation if your router or access server is connected to another vendor's equipment across a Frame Relay network. IETF encapsulation is supported either at the interface level or on a per-virtual circuit basis.

We recommend that you shut down the interface prior to changing encapsulation types. Although this is not required, shutting down the interface ensures the interface is reset for the new encapsulation.








Enable Frame Relay Encapsulation on an Interface

To set Frame Relay encapsulation at the interface level, use the following commands beginning in global configuration mode:


1. interface type number Specify the interface, and enter interface configuration mode.

2. encapsulation frame-relay [ietf]

Enable Frame Relay, and specify the encapsulation method.

Frame Relay supports encapsulation of all supported protocols in conformance with RFC 1490, allowing interoperability between multiple vendors. Use the Internet Engineering Task Force (IETF) form of Frame Relay encapsulation if your router or access server is connected to another vendor's equipment across a Frame Relay network. IETF encapsulation is supported either at the interface level or on a per-virtual circuit basis.

We recommend that you shut down the interface prior to changing encapsulation types. Although this is not required, shutting down the interface ensures the interface is reset for the new encapsulation.

Configure Dynamic or Static Address Mapping

Dynamic address mapping uses Frame Relay Inverse ARP to request the next hop protocol address for a specific connection, given its known DLCI. Responses to Inverse ARP requests are entered in an address-to-DLCI mapping table on the router or access server; the table is then used to supply the next hop protocol address or the DLCI for outgoing traffic.

Inverse ARP is enabled by default for all protocols it supports, but can be disabled for specific protocol-DLCI pairs. As a result, you can use dynamic mapping for some protocols and static mapping for other protocols on the same DLCI. You can explicitly disable Inverse ARP for a protocol-DLCI pair if you know that the protocol is not supported on the other end of the connection. See the "Disable or Reenable Frame Relay Inverse ARP" section later in this chapter for more information.

Configure Dynamic Mapping

Inverse ARP is enabled by default for all protocols enabled on the physical interface. Packets are not sent out for protocols that are not enabled on the interface.

Because Inverse ARP is enabled by default, no additional command is required to configure dynamic mapping on an interface.

Configure Static Mapping


A static map links a specified next hop protocol address to a specified DLCI. Static mapping removes the need for Inverse ARP requests; when you supply a static map, Inverse ARP is automatically disabled for the specified protocol on the specified DLCI.

You must use static mapping if the router at the other end either does not support Inverse ARP at all or does not support Inverse ARP for a specific protocol that you want to use over Frame Relay.

To establish static mapping according to your network needs, use one of the following commands in interface configuration mode:

Command Purpose

frame-relay map protocol protocol-address dlci [broadcast] [ietf] [cisco]

Define the mapping between a next hop protocol address and the DLCI used to connect to the address.

frame-relay map clns dlci [broadcast]

Define a DLCI used to send International Organization for Standardization (ISO) Connectionless Network Service (CLNS) frames.

frame-relay map bridge dlci [broadcast] [ietf] Define a DLCI used to connect to a bridge.

The supported protocols and the corresponding keywords to enable them are as follows:

IP—ip DECnet—decnet AppleTalk—appletalk XNS—xns Novell IPX—ipx VINES—vines ISO CLNS—clns

You can greatly simplify the configuration for the Open Shortest Path First (OSPF) protocol by adding the optional broadcast keyword when doing this task. See the frame-relay map command description in the Wide-Area Networking Command Reference and the examples at the end of this chapter for more information about using the broadcast keyword.

For examples of how to establish static address mapping, see the "Static Address Mapping Examples" section later in this chapter.

Configure the LMI

Beginning with Cisco IOS Release 11.2, the software supports Local Management Interface (LMI) autosense, which enables the interface to determine the LMI type supported by the switch.



Support for LMI autosense means that you are no longer required to configure the Local Management Interface (LMI) explicitly.

For information on using Enhanced Local Management Interface with traffic shaping, see "Configure Frame Relay Traffic Shaping" later in this chapter.

Allow LMI Autosense to Operate

LMI autosense is active in the following situations:

The router is powered up or the interface changes state to up. The line protocol is down but the line is up. The interface is a Frame Relay DTE. The LMI type is not explicitly configured.

Status Request

When LMI autosense is active, it sends out a full status request, in all three LMI flavors, to the switch. The order is ANSI, ITU, cisco but is done in rapid succession. Unlike previous software capability, we can now listen in on both DLCI 1023 (cisco LMI) and DLCI 0 (ANSI and ITU) simultaneously.

Status Messages

One or more of the status requests will elicit a reply (status message) from the switch. The router will decode the format of the reply and configure itself automatically. If more than one reply is received, the router will configure itself with the type of the last received reply. This is to accommodate intelligent switches that can handle multiple formats simultaneously.

LMI Autosense

If LMI autosense is unsuccessful, an intelligent retry scheme is built in. Every N391 interval (default is 60 seconds, which is 6 keep exchanges at 10 seconds each), LMI autosense will attempt to ascertain the LMI type. For more information about N391, see the frame-relay lmi-n391dte command in the "Frame Relay Commands" chapter of the Wide-Area Networking Command Reference.

The only visible indication to the user that LMI autosense is underway is when debug frame lmi is turned on. Every N391 interval, the user will now see three rapid status enquiries coming out of the serial interface. One in ANSI, one in ITU and one in cisco LMI-type.

Configuration Options


No configuration options are provided; this is transparent to the user. You can turn off LMI autosense by explicitly configuring an LMI type. The LMI type must be written into NVRAM so that next time the router powers up, LMI autosense will be inactive. At the end of autoinstall, a frame-relay lmi-type xxx statement is included within the interface configuration. This configuration is not automatically written to NVRAM; you must explicitly write the configuration to NVRAM by using the copy system:running-config or copy nvram:startup-config commands.

Explicitly Configure the LMI

Our Frame Relay software supports the industry-accepted standards for addressing the Local Management Interface (LMI), including the Cisco specification. If you want to configure the LMI and thus deactivate LMI autosense, perform the tasks in the following sections. The tasks in the first two sections are required if you choose to configure the LMI.

Set the LMI Type Set the LMI Keepalive Interval Set the LMI Polling and Timer Intervals

Set the LMI Type

If the router or access server is attached to a public data network (PDN), the LMI type must match the type used on the public network. Otherwise, the LMI type can be set to suit the needs of your private Frame Relay network.

You can set one of three types of LMIs on our devices: ANSI T1.617 Annex D, Cisco, and ITU-T Q.933 Annex A. To do so, use the following commands beginning in interface configuration mode:


1. frame-relay lmi-type {ansi | cisco | q933a} Set the LMI type.

2. end Exit configuration mode.

3. copy nvram:startup-config destination Write the LMI type to NVRAM.

For an example of how to set the LMI type, see the "Pure Frame Relay DCE Example" section later in this chapter.

Set the LMI Keepalive Interval





A keepalive interval must be set to configure the LMI. By default, this interval is 10 seconds and, per the LMI protocol, must be less than the corresponding interval on the switch. To set the keepalive interval, use the following command in interface configuration mode:

Command Purpose

keepalive number Set the keepalive interval.

To disable keepalives on networks that don't utilize LMI, use the no keepalive interface configuration command.For an example of how to specify an LMI keepalive interval, see the "Two Routers in Static Mode Example" section later in this chapter.

Understand Frame Relay Subinterfaces

Frame Relay subinterfaces provide a mechanism for supporting partially meshed Frame Relay networks. Most protocols assume transitivity on a logical network; that is, if station A can talk to station B, and station B can talk to station C, then station A should be able to talk to station C directly. Transitivity is true on LANs, but not on Frame Relay networks unless A is directly connected to C.

Additionally, certain protocols such as AppleTalk and transparent bridging cannot be supported on partially meshed networks because they require "split horizon," in which a packet received on an interface cannot be transmitted out the same interface even if the packet is received and transmitted on different virtual circuits.

Configuring Frame Relay subinterfaces ensures that a single physical interface is treated as multiple virtual interfaces. This capability allows us to overcome split horizon rules. Packets received on one virtual interface can now be forwarded out another virtual interface, even if they are configured on the same physical interface.

Subinterfaces address the limitations of Frame Relay networks by providing a way to subdivide a partially meshed Frame Relay network into a number of smaller, fully meshed (or point-to-point) subnetworks. Each subnetwork is assigned its own network number and appears to the protocols as if it is reachable through a separate interface. (Note that point-to-point subinterfaces can be unnumbered for use with IP, reducing the addressing burden that might otherwise result.)

For example, suppose you have a five-node Frame Relay network (see Figure 10) that is partially meshed (Network A). If the entire network is viewed as a single subnetwork (with a single network number assigned), most protocols assume that node A can transmit a packet directly to node E, when in fact it must be relayed through nodes C and D. This network can be made to work with certain protocols (for example, IP) but will not work at all with other protocols (for example, AppleTalk) because nodes C and D will not relay the packet out the same interface on which it was received. One way to make this network work fully is to create a fully meshed



network (Network B), but doing so requires a large number of PVCs, which may not be economically feasible.

Using subinterfaces, you can subdivide the Frame Relay network into three smaller subnetworks (Network C) with separate network numbers. Nodes A, B, and C are connected to a fully meshed network, and nodes C and D, as well as nodes D and E are connected via point-to-point networks. In this configuration, nodes C and D can access two subinterfaces and can therefore forward packets without violating split horizon rules. If transparent bridging is being used, each subinterface is viewed as a separate bridge port.

Figure 10 Using Subinterfaces to Provide Full Connectivity on a Partially Meshed Frame Relay Network

Define Frame Relay Subinterfaces

To configure subinterfaces on a Frame Relay network, use the following commands beginning in global configuration mode:


1. interface type number.subinterface-number {multipoint | point-to-point}

Create a point-to-point or multipoint subinterface.

2. encapsulation frame-relay Configure Frame Relay encapsulation on the serial interface.

Subinterfaces can be configured for multipoint or point-to-point communication. (There is no default.)

Define Subinterface Addressing

For point-to-point subinterfaces, the destination is presumed to be known and is identified or implied in the frame-relay interface-dlci command. For multipoint subinterfaces, the destinations can be dynamically resolved through the use of Frame Relay Inverse ARP or can be statically mapped through the use of the frame-relay map command.

Addressing on Point-to-Point Subinterfaces

If you specified a point-to-point subinterface in the previous procedure, use the following command in subinterface configuration mode:

Command Purpose

frame-relay interface-dlci dlci Associate the selected point-to-point subinterface with a DLCI.

Note This command is typically used on subinterfaces; however, it can also be applied to main interfaces. The frame-relay interface-dlci command is used to enable routing protocols on main interfaces that are configured to use Inverse ARP. This command is also helpful for assigning a specific class to a single PVC on a multipoint subinterface.

If you define a subinterface for point-to-point communication, you cannot reassign the same subinterface number to be used for multipoint communication without first rebooting the router or access server. Instead, you can simply avoid using that subinterface number and use a different subinterface number instead.

Addressing on Multipoint Subinterfaces

If you specified a multipoint subinterface in under "Define Frame Relay Subinterfaces," perform the tasks in one or both of the following sections:

Accept Inverse ARP for Dynamic Address Mapping on Multipoint Subinterfaces Configure Static Address Mapping on Multipoint Subinterfaces

You can configure some protocols for dynamic address mapping and others for static address mapping.

Accept Inverse ARP for Dynamic Address Mapping on Multipoint Subinterfaces

Dynamic address mapping uses Frame Relay Inverse ARP to request the next hop protocol address for a specific connection, given a DLCI. Responses to Inverse ARP requests are entered in an address-to-DLCI mapping table on the router or access server; the table is then used to supply the next hop protocol address or the DLCI for outgoing traffic.

Since the physical interface is now configured as multiple subinterfaces, you must provide information that distinguishes a subinterface from the physical interface and associates a specific subinterface with a specific DLCI.

To associate a specific multipoint subinterface with a specific DLCI, use the following command in interface configuration mode:

Command Purpose

frame-relay interface-dlci dlci Associate a specified multipoint subinterface with a DLCI.

Inverse ARP is enabled by default for all protocols it supports, but can be disabled for specific protocol-DLCI pairs. As a result, you can use dynamic mapping for some protocols and static mapping for other protocols on the same DLCI. You can explicitly disable Inverse ARP for a protocol-DLCI pair if you know the protocol is not supported on the other end of the connection. See the "Disable or Reenable Frame Relay Inverse ARP" section later in this chapter for more information.

Because Inverse ARP is enabled by default for all protocols that it supports, no additional command is required to configure dynamic address mapping on a subinterface.

For an example of configuring Frame Relay multipoint subinterfaces with dynamic address mapping, see the "Frame Relay Multipoint Subinterface with Dynamic Addressing Example" section.

Configure Static Address Mapping on Multipoint Subinterfaces








Command Purpose


Define the mapping between a next hop protocol address and the DLCI used to connect to that address.

frame-relay map clns dlci [broadcast] Define a DLCI used to send ISO CLNS frames.




The broadcast keyword is required for routing protocols such as OSI protocols and the Open Shortest Path First (OSPF) protocol. See the frame-relay map command description in the Wide-Area Networking Command Reference and the examples at the end of this chapter for more information about using the broadcast keyword.

Configure Dynamic or Static Address Mapping

Dynamic address mapping uses Frame Relay Inverse ARP to request the next hop protocol address for a specific connection, given its known DLCI. Responses to Inverse ARP requests are

entered in an address-to-DLCI mapping table on the router or access server; the table is then used to supply the next hop protocol address or the DLCI for outgoing traffic.

Inverse ARP is enabled by default for all protocols it supports, but can be disabled for specific protocol-DLCI pairs. As a result, you can use dynamic mapping for some protocols and static mapping for other protocols on the same DLCI. You can explicitly disable Inverse ARP for a protocol-DLCI pair if you know that the protocol is not supported on the other end of the connection. See the "Disable or Reenable Frame Relay Inverse ARP" section later in this chapter for more information.

Configure Dynamic Mapping

Inverse ARP is enabled by default for all protocols enabled on the physical interface. Packets are not sent out for protocols that are not enabled on the interface.

Because Inverse ARP is enabled by default, no additional command is required to configure dynamic mapping on an interface.

Configure Static Mapping


You must use static mapping if the router at the other end either does not support Inverse ARP at all or does not support Inverse ARP for a specific protocol that you want to use over Frame Relay. To establish static mapping according to your network needs, use one of the following commands in interface configuration mode:

Command Purpose



Define the mapping between a next hop protocol address and the DLCI used to connect to the address.

frame-relay map clns dlci [broadcast]

Define a DLCI used to send International Organization for Standardization (ISO) Connectionless Network Service (CLNS) frames.




You can greatly simplify the configuration for the Open Shortest Path First (OSPF) protocol by adding the optional broadcast keyword when doing this task. See the frame-relay map command description in the Wide-Area Networking Command Reference and the examples at the end of this chapter for more information about using the broadcast keyword.

Configure the LMI

Beginning with Cisco IOS Release 11.2, the software supports Local Management Interface (LMI) autosense, which enables the interface to determine the LMI type supported by the switch. Support for LMI autosense means that you are no longer required to configure the Local Management Interface (LMI) explicitly.

Allow LMI Autosense to Operate

LMI autosense is active in the following situations:

The router is powered up or the interface changes state to up. The line protocol is down but the line is up. The interface is a Frame Relay DTE. The LMI type is not explicitly configured.

Status Request

When LMI autosense is active, it sends out a full status request, in all three LMI flavors, to the switch. The order is ANSI, ITU, cisco but is done in rapid succession. Unlike previous software capability, we can now listen in on both DLCI 1023 (cisco LMI) and DLCI 0 (ANSI and ITU) simultaneously.

Status Messages

One or more of the status requests will elicit a reply (status message) from the switch. The router will decode the format of the reply and configure itself automatically. If more than one reply is received, the router will configure itself with the type of the last received reply. This is to accommodate intelligent switches that can handle multiple formats simultaneously.

LMI Autosense

If LMI autosense is unsuccessful, an intelligent retry scheme is built in. Every N391 interval (default is 60 seconds, which is 6 keep exchanges at 10 seconds each), LMI autosense will attempt to ascertain the LMI type. For more information about N391, see the frame-relay lmi-n391dte command in the "Frame Relay Commands" chapter of the Wide-Area Networking Command Reference.

The only visible indication to the user that LMI autosense is underway is when debug frame lmi is turned on. Every N391 interval, the user will now see three rapid status enquiries coming out of the serial interface. One in ANSI, one in ITU and one in cisco LMI-type.

Configuration Options

No configuration options are provided; this is transparent to the user. You can turn off LMI autosense by explicitly configuring an LMI type. The LMI type must be written into NVRAM so that next time the router powers up, LMI autosense will be inactive. At the end of autoinstall, a frame-relay lmi-type xxx statement is included within the interface configuration. This configuration is not automatically written to NVRAM; you must explicitly write the configuration to NVRAM by using the copy system:running-config or copy nvram:startup-config commands.

Explicitly Configure the LMI

Our Frame Relay software supports the industry-accepted standards for addressing the Local Management Interface (LMI), including the Cisco specification. If you want to configure the LMI and thus deactivate LMI autosense, perform the tasks in the following sections. The tasks in the first two sections are required if you choose to configure the LMI.

Set the LMI Type


Set the LMI Keepalive Interval Set the LMI Polling and Timer Intervals

Set the LMI Type

If the router or access server is attached to a public data network (PDN), the LMI type must match the type used on the public network. Otherwise, the LMI type can be set to suit the needs of your private Frame Relay network.

You can set one of three types of LMIs on our devices: ANSI T1.617 Annex D, Cisco, and ITU-T Q.933 Annex A. To do so, use the following commands beginning in interface configuration mode:


1. frame-relay lmi-type {ansi | cisco | q933a} Set the LMI type.

2. end Exit configuration mode.

3. copy nvram:startup-config destination Write the LMI type to NVRAM.

Set the LMI Keepalive Interval

A keepalive interval must be set to configure the LMI. By default, this interval is 10 seconds and, per the LMI protocol, must be less than the corresponding interval on the switch. To set the keepalive interval, use the following command in interface configuration mode:

Command Purpose

keepalive number Set the keepalive interval.

To disable keepalives on networks that don't utilize LMI, use the no keepalive interface configuration command.

Configure Frame Relay Switched Virtual Circuits

Access to Frame Relay networks is made through private leased lines at speeds ranging from 56 kbps to 45 Mbps. Frame Relay is a connection-oriented, packet-transfer mechanism that establishes virtual circuits between endpoints.

Switched Virtual Circuits



Switched virtual circuits (SVCs) allow access through a Frame Relay network by setting up a path to the destination endpoints only when the need arises and tearing down the path when it is no longer needed.

SVCs can coexist with PVCs in the same sites and routers. For example, routers at remote branch offices might set up PVCs to the central headquarters for frequent communication, but set up SVCs with each other as needed for intermittent communication. As a result, any-to-any communication can be set up without any-to-any PVCs.

On SVCs, quality of service (QOS) elements can be specified on a call-by-call basis to request network resources.

SVC support is offered in the Enterprise image on Cisco platforms that include a serial or HSSI interface.

You must have the following services before Frame Relay SVCs can operate:

Frame Relay SVC support by the service provider—The service provider's switch must be capable of supporting SVC operation.

Physical loop connection—A leased line or dedicated line must exist between the router (DTE) and the local Frame Relay switch.

SVC Operation

SVC operation requires that the Data Link layer (Layer 2) be set up, running ITU-T Q.922 Link Access Procedures to Frame mode bearer services (LAPF), prior to signalling for an SVC. Layer 2 sets itself up as soon as SVC support is enabled on the interface, if both the line and the line protocol are up. When the SVCs are configured and demand for a path occurs, the Q.933 signalling sequence is initiated. Once the SVC is set up, data transfer begins.

Q.922 provides a reliable link layer for Q.933 operation. All Q.933 call control information is transmitted over DLCI 0; this DLCI is also used for the management protocols specified in ANSI T1.617 Annex D or Q.933 Annex A.

You must enable SVC operation at the interface level. Once it is enabled at the interface level, it is enabled on any subinterfaces on that interface. One signalling channel, DLCI 0, is set up for the interface, and all SVCs are controlled from the physical interface.

Enabling Frame Relay SVC Service

To enable Frame Relay SVC service and set up SVCs, perform the tasks in the following sections. The Subinterface’s tasks are not required, but offer additional flexibility for SVC configuration and operation. The LAPF tasks are not required and not recommended unless you understand thoroughly the impacts on your network.

Configure SVCs on a Physical Interface

To enable SVC operation on a Frame Relay interface, use the following commands beginning in global configuration mode:


1. interface type number Specify the physical interface.

2. ip address ip-address mask Specify the interface IP address, if needed.

3. encapsulation frame-relay Enable Frame Relay encapsulation on the interface.

4. map-group group-name Assign a map group to the interface.

5. frame-relay svc Enable Frame Relay SVC support on the interface.

Map-group details are specified with the map-list command.

Configure SVCs on a Subinterface

To configure Frame Relay SVCs on a Subinterface, complete all the commands in the previous section, except assigning a the map group. After the physical interface is configured, use the following commands beginning in global configuration mode:


1. interface type number.subinterface-number {multipoint | point-to-point}

Specify a subinterface of the main interface configured for SVC operation.

2. ip address ip-address mask Specify the subinterface IP address, if needed.

3. map-group group-name Assign a map group to the subinterface.

Define Subinterface Addressing

For point-to-point Subinterface’s, the destination is presumed to be known and is identified or implied in the frame-relay interface-dlci command. For multipoint Subinterface’s, the destinations can be dynamically resolved through the use of Frame Relay Inverse ARP or can be statically mapped through the use of the frame-relay map command.

Addressing on Point-to-Point Subinterface’s

If you specified a point-to-point Subinterface’s in the previous procedure, use the following command in Subinterface’s configuration mode:

Command Purpose

frame-relay interface-dlci dlci Associate the selected point-to-point subinterface with a DLCI.

Note This command is typically used on subinterfaces; however, it can also be applied to main interfaces. The frame-relay interface-dlci command is used to enable routing protocols on main interfaces that are configured to use Inverse ARP. This command is also helpful for assigning a specific class to a single PVC on a multipoint subinterface.

For an explanation of the many available options, refer to this command in the Wide-Area Networking Command Reference. For an example of how to associate a DLCI with a subinterface, see the section "Subinterface Examples" later in this chapter.

If you define a subinterface for point-to-point communication, you cannot reassign the same subinterface number to be used for multipoint communication without first rebooting the router or access server. Instead, you can simply avoid using that subinterface number and use a different subinterface number instead.

Addressing on Multipoint Subinterfaces

If you specified a multipoint subinterface in under "Define Frame Relay Subinterfaces," perform the tasks in one or both of the following sections:

Accept Inverse ARP for Dynamic Address Mapping on Multipoint Subinterfaces Configure Static Address Mapping on Multipoint Subinterfaces

You can configure some protocols for dynamic address mapping and others for static address mapping.

Accept Inverse ARP for Dynamic Address Mapping on Multipoint Subinterface’s

Dynamic address mapping uses Frame Relay Inverse ARP to request the next hop protocol address for a specific connection, given a DLCI. Responses to Inverse ARP requests are entered in an address-to-DLCI mapping table on the router or access server; the table is then used to supply the next hop protocol address or the DLCI for outgoing traffic.

Since the physical interface is now configured as multiple Subinterface’s, you must provide information that distinguishes a Subinterface’s from the physical interface and associates a specific Subinterface’s with a specific DLCI.




To associate a specific multipoint Subinterface with a specific DLCI, use the following command in interface configuration mode:

Command Purpose

frame-relay interface-dlci dlci Associate a specified multipoint subinterface with a DLCI.

Inverse ARP is enabled by default for all protocols it supports, but can be disabled for specific protocol-DLCI pairs. As a result, you can use dynamic mapping for some protocols and static mapping for other protocols on the same DLCI. You can explicitly disable Inverse ARP for a protocol-DLCI pair if you know the protocol is not supported on the other end of the connection. See the "Disable or Reenable Frame Relay Inverse ARP" section later in this chapter for more information.

Because Inverse ARP is enabled by default for all protocols that it supports, no additional command is required to configure dynamic address mapping on a subinterface.

For an example of configuring Frame Relay multipoint subinterfaces with dynamic address mapping, see the "Frame Relay Multipoint Subinterface with Dynamic Addressing Example" section.

Configure Static Address Mapping on Multipoint Subinterfaces




Command Purpose


Define the mapping between a next hop protocol address and the DLCI used to connect to that address.



frame-relay map clns dlci [broadcast] Define a DLCI used to send ISO CLNS frames.


Disable or Reenable Frame Relay Inverse ARP

Frame Relay Inverse ARP is a method of building dynamic address mappings in Frame Relay networks running AppleTalk, Banyan VINES, DECnet, IP, Novell IPX, and XNS. Inverse ARP allows the router or access server to discover the protocol address of a device associated with the virtual circuit.

Inverse ARP creates dynamic address mappings, as contrasted with the frame-relay map command, which defines static mappings between a specific protocol address and a specific DLCI (see the section "Configure Dynamic or Static Address Mapping" earlier in this chapter for more information).

Inverse ARP is enabled by default but can be disabled explicitly for a given protocol and DLCI pair. Disable or reenable Inverse ARP under the following conditions:

Disable Inverse ARP for a selected protocol and DLCI pair when you know that the protocol is not supported on the other end of the connection.

Reenable Inverse ARP for a protocol and DLCI pair if conditions or equipment change and the protocol is then supported on the other end of the connection.

Note If you change from a point-to-point subinterface to a multipoint subinterface, then change the subinterface number. Frame Relay Inverse ARP will be on by default, and no further action is required.

You do not need to enable or disable Inverse ARP if you have a point-to-point interface, because there is only a single destination and discovery is not required.

To select Inverse ARP or disable it, use one of the following commands in interface configuration mode:

Command Purpose

frame-relay inverse-arp Enable Frame Relay Inverse ARP for a specific protocol and


protocol dlci DLCI pair, only if it was previously disabled.

no frame relay inverse-arp protocol dlci

Disable Frame Relay Inverse ARP for a specific protocol and DLCI pair.

Basic Subinterface Examples

In the following example, Subinterface’s 1 models a point-to-point subnet and Subinterface’s 2 models a multipoint subnet.

interface serial 0

encapsulation frame-relay

interface serial 0.1 point-to-point

ip address 10.0.1.1 255.255.255.0

frame-relay interface-dlci 42

!

interface serial 0.2 multipoint

ip address 10.0.2.1 255.255.255.0

frame-relay map 10.0.2.2 18

Frame Relay Multipoint Subinterface with Dynamic Addressing Example

The following example configures two multipoint Subinterface’s for dynamic address resolution. Each subinterfaces is provided with an individual protocol address and subnet mask, and the frame-relay interface-dlci command associates the subinterfaces with a specified DLCI. Addresses of remote destinations for each multipoint subinterface will be resolved dynamically.

interface Serial0

no ip address


frame-relay lmi-type ansi

interface Serial0.103 multipoint

ip address 172.21.177.18 255.255.255.0


!

interface Serial0.104 multipoint

ip address 172.21.178.18 255.255.255.0


Enhanced Local Management Interface Example

Figure 12 illustrates a Cisco switch and a Cisco router, both configured with the Enhanced Local Management Interface feature enabled. The switch sends QOS information to the router, which uses it for traffic rate enforcement.

Figure 12 Enhanced Local Management Interface—Sent between the Cisco Switch and the Cisco Router

This configuration example shows a Frame-Relay interface enabled with QOS autosense. The router receives messages from the Cisco switch, which is also configured with QOS autosense enabled. When Enhanced Local Management Interface is configured in conjunction with traffic shaping, the router will receive congestion information through BECN or Router ForeSight congestion signaling and reduce its output rate to the value specified in the traffic shaping configuration.

interface serial0


no ip address



frame-relay traffic-shaping

frame-relay qos-autosense

!

interface serial0.1 point-to-point

no ip address


Configuration Providing Backward Compatibility Example

The following configuration provides backward compatibility and interoperability with earlier versions that are not compliant with RFC 1490. The ietf keyword is used to generate RFC 1490 traffic. This configuration is possible because of the flexibility provided by separately defining each map entry.


frame-relay map ip 131.108.123.2 48 broadcast ietf

! interoperability is provided by IETF encapsulation

frame-relay map ip 131.108.123.3 49 broadcast ietf

frame-relay map ip 131.108.123.7 58 broadcast

! this line allows the router to connect with a ! device running an older version of software

frame-relay map decnet 21.7 49 broadcast

Configure IETF based on map entries and protocol for more flexibility. Use this method of configuration for backward compatibility and interoperability.

Booting from a Network Server over Frame Relay Example

When booting from a Trivial File Transfer Protocol (TFTP) server over Frame Relay, you cannot boot from a network server via a broadcast. You must boot from a specific TFTP host. Also, a frame-relay map command must exist for the host that you will boot from.

For example, if file gs3-bfx is to be booted from a host with IP address 131.108.126.2, the following commands would need to be in the configuration:

boot system gs3-bfx 131.108.126.2

!

interface Serial 0


frame-relay map IP 131.108.126.2 100 broadcast

The frame-relay map command is used to map an IP address into a DLCI address. To boot over Frame Relay, you must explicitly give the address of the network server to boot from, and a frame-relay map entry must exist for that site. For example, if file gs3-bfx.83-2.0 is to be booted from a host with IP address 131.108.126.111, the following commands must be in the configuration:

boot system gs3-bfx.83-2.0 131.108.13.111

!

interface Serial 1

ip address 131.108.126.200 255.255.255.0



In this case, 100 is the DLCI that can get to host 131.108.126.111.

The remote router must have the following frame-relay map entry:


This entry allows the remote router to return a boot image (from the network server) to the router booting over Frame Relay. Here, 101 is a DLCI of the router being booted.

PVC Switching Configuration Example

You can configure your router as a dedicated, DCE-only Frame Relay switch. Switching is based on DLCIs. The incoming DLCI is examined, and the outgoing interface and DLCI are

determined. Switching takes place when the incoming DLCI in the packet is replaced by the outgoing DLCI, and the packet is sent out the outgoing interface.

In Figure 13, the router switches two PVCs between interface serial 1 and 2. Frames with DLCI 100 received on serial 1 will be transmitted with DLCI 200 on serial 2.

Figure 13 PVC Switching Configuration

Configuration for Router A

frame-relay switching

!

interface Ethernet0

ip address 131.108.160.58 255.255.255.0

!

interface Serial1

no ip address


keepalive 15


frame-relay intf-type dce

frame-relay route 100 interface Serial2 200


clockrate 2000000

!

interface Serial2


keepalive 15





clockrate 64000

Pure Frame Relay DCE Example

Using the PVC switching feature, it is possible to build an entire Frame Relay network using our routers. In Figure 14, Router A and Router C act as Frame Relay switches implementing a two-


node network. The standard Network-to-Network Interface (NNI) signaling protocol is used between Router A and Router C.

Figure 14 Frame Relay DCE Configuration

Configuration for Router A


!

interface ethernet 0

no ip address

shutdown :Interfaces not in use may be shut down; shut down is not required.

!


no ip address

shutdown

!


no ip address

shutdown

!


no ip address

shutdown

!

interface serial 0

ip address 131.108.178.48 255.255.255.0

shutdown

!

interface serial 1

no ip address




frame-relay route 100 interface serial 2 200

!

interface serial 2

no ip address


frame-relay intf-type nni

frame-relay lmi-type q933a


clockrate 2048000

!

interface serial 3

no ip address

shutdown

Configuration for Router C


!


no ip address


!

interface ethernet1

no ip address

shutdown

!


no ip address

shutdown

!


no ip address

shutdown

!

interface serial 0

ip address 131.108.187.84 255.255.255.0

shutdown

!

interface serial 1

no ip address




!

interface serial 2

no ip address


frame-relay intf-type nni

frame-relay lmi-type q933a


!

interface serial 3

no ip address

shutdown

Hybrid DTE/DCE PVC Switching Example

Routers can also be configured as hybrid DTE/DCE Frame Relay switches, as shown in Figure 15.

Figure 15 Hybrid DTE/DCE PVC Switching

In the following example, Router B acts as a hybrid DTE/DCE Frame Relay switch. It can switch frames between the two DCE ports and between a DCE port and a DTE port. Traffic from the Frame Relay network can also be terminated locally. In the example, three PVCs are defined, as follows:

Serial 1, DLCI 102 to serial 2, DLCI 201—DCE switching Serial 1, DLCI 103 to serial 3, DLCI 301—DCE/DTE switching Serial 2, DLCI 203 to serial 3, DLCI 302—DCE/DTE switching

DLCI 400 is also defined for locally terminated traffic.

Configuration for Router B


!


ip address 131.108.123.231 255.255.255.0

!


ip address 131.108.5.231 255.255.255.0

!

interface serial 0



no ip address


!

interface serial 1

no ip address





!

interface serial 2

no ip address





!

interface serial 3

ip address 131.108.111.231






ATM

ATM is used between the Frame Rely switch and the edged of the network.

DTE--DCE-[ATM—ATM]—DCE—DTE

ATM- Uses cells which are 53 bytes long.

ATM- Asynchronous Transfer mode (ATM)Better QOS

Services Internetworking = ATM between two FR switches.

How Frame Relay Switches interoperate using ATM in a Frame Relay VC to an ATM VC and BACK to a FR VC is Called FRF5

FRF.5 – Defines how a Frame Relay switch can convert form FR VC to ATM VC and Back to a FR VC. Which is totally transparent to the DCE (Routers)

FRF.8 – Service internetworking defines how two routers communicate when one router is connected to a FR network and the other is connected to an ATM network.

IP Frames/Packets Protocols

L3 protocol IP only

L2 protocol Ethernet

L2- Data Link Protocol/MAC address

Ethernet – Uses ARP to find interface the other MAC address interface

Frame Relay – Uses DLCI to identify the other side

ATM -

HDLC/PPP Serial – Uses Nothing Because there is only one other side.

ISDN

ISDN- Integrated Services Digital Network (ISDN)

ISDN Protocols

E-Series – Telephone Network and ISDN

E- 163

E – 163

I-Series – ISDN concepts, aspects and Interfaces

I-100

I-400

Q-Series – Switching & Signaling

Used to establish links ----Q.921 –

LAPD –Link Access Procedure on the D Channel .

Used to Tear-down links----Q-931 – ISDN network Layer

Q.921 – is used as a Data link Layer 2 protocol across an ISDN D channel (LAPD) – Encapsulates signaling requests

Q-931- Signaling is used to Set-up and Tear down a circuit using Layer 3 Setup and Tear down Messages.

SPID – Service Profile Identifier – Used to perform authentication.

ISDN function groups & Reference Points

Function Group - Is a Set of function implemented by a device and software.

Function Groups for ISDN

NT1 -------- ISDN interface

TE1 --------

TE2 --------

NT2 – Used outside the North American network connects to a NT1 which is used in the North American networks.

Reference Point – The interface between two function groups including cabling details.

Reference Points

R-------------TE2 AND TA

S-------------

T-------------

U------------

S/T

Example:

[PC] ---(R)-[MODEM](U)[TELCO ISDN NETWORK]

NT – Network Termination

TE – Terminal Termination

---- = BRI Circuit

U = Reference Point is used with interfaces with NT1 and TE1

[Router]----------- (U) ---------- [TELCO ISDN NETWORK]

Router with a BRI-U interface already has all the equipment needed to communicate with the ISDN Telco Network. So only a U reference point is needed.

[Router with (BRI-U interface)] -------------(U)-----------[TELCO ISDN NETWORK]

BRI-U interface has the built in ISDN interface.

U = Reference Point is used with interfaces with NT1 and TE1 built in.

[Router with (BRI-S/T interface)]--(ST)--(NT1)----(U)----- [TELCO ISDN NETWORK]

Router with a BRI-S/T interface needs an NT1 connection to the ISDN Network.

[Router with a Serial interface needs an]----(R)---(TA)---(S/T)---(NT1)---U---[ISDN NT]

TA- Terminal Adapter

Serial interfaces need additional equipment to be able to communicate on the ISDN network.

Non North American Network (Canada)

[Router with a Serial interface needs an]----(S) ---(NT2) ---(T)---(NT1)---U---[ISDN NT]

NT2 is only used when the circuit is not in a North American network.

Rules ISDN connections

BRI U Router Interface----------U---------- [TELCO ISDN NETWORK]

BRI-S/T Router Interface ----------NT1-----------U------------ [TELCO ISDN NETWORK]

[Router with (BRI-S/T interface)]--(S/T)—[NT1] ---- (U) ---- [TELCO ISDN NETWORK]

[Router with a Serial interface needs an]----(R) --- (TA) ---(S/T) --- (NT1) ---U--- [ISDN NT]

Non-North American ISDN network

[Router with a Serial interface needs an]----(S) --- (NT2)---(T)---(NT1)---U---[ISDN NT]

PRI’s have no reference Point or Function Groups.

Encoding

ISDN PRI in North America is base on a digital circuit T1 circuit uses two different encoding schemes.

Alternative Mark Invasion (AMI)

Binary 8 with Zero Substitution (8BZS)

E1 – uses High Density Bipolar 3 (HDB3) encoding schemes.

PRI Framing

The two framing option are based on T1or E1 specifications.

T1 – 24 – 64kbps DS) channels plus an 8-kbps management used by the telco which gives you a total speed of 1.544mbps.

E1 – 32 64-kbps ch = 2.048 mbps.

The two options for framing on T1 are to use either:

Extended Super Frame (ESF) (More commonly used for T1 Today)

Older- Super Frame (SF)

ISDN Dial-on Demand router DDR

Multilink PPP (MLP) which allows multiple B channels to be connected to the same remote sit.

DDR can be used to cause the router to dial or to receive a dialed call on asynchronous Serial Interfaces, Synchronous Serial interface, and ISDN BRI and PRI interfaces.

Two Types:

Legacy DDRDDR dialer profiles

DDR does not dial until some traffic is directed (routed) out the dial interface.

Legacy DDR commands

Router (config-int) IP route 172.16.3.0 255.255.255.0 172.18.2.1

Via

Trigger the dial which is configured on the router.

Cisco calls packets that are worthy of causing the device to dial Interesting packets .

Two different methods can be used to define interesting packets

All Interesting Packets (Layer 3)

Packets matching access control lists (ACL) Dialer group 1 = Interesting Traffic.

Integrated Services Digital Network (ISDN)

To Test if an ISDN Dial-up connection is working use the Following Tools:

Dial-up Connection (Layer 1) Physical Test

Ping –ICMP (Layer 2) –Testes Layer 2 (Data-Link) Connectivity

Telnet - (Layer 3 Protocol) – Tests Layer 3 (Network) Connectivity

TraceRoute (Layer 2) – Tests and Checks the Route a packet takes to a Destination.

Layer 2 (Data-Link) Utility Identifies the Layer 2 Path that a Packet Takes from a Source Device to a Destination Device. Layer 2 TraceRoute supports only unicasts Source and Destination MAC Address.

The TraceRoute Command is used to Discover the Route that the Packet takes when Traveling to their Destination. The device (PC, SWITCH, and ROUTER) Sends out a Sequence of User Datagrams Protocol (UDP) data grams to an invalid port address at the Remote Host.

Testing an ISDN Connection use:

Start a Dialup Connection to test Layer 1

Ping (ICMP) to test Layer 2 (Ping is more Dependable then Trace Route)

Telnet to Test Layer 3 connectivity

Fundamentals of WANs CH 4

1) Which of the following best describes the main function of OSI Layer 1 protocols?

Deliver of bits form on device to another

2) Which of the following typically connects to a V.35 or RS-232 end of a cable when cabling leased line?

B) CSU/DSU

3) Which of the following typically connects to a four-wire line provided by a teleco?

B) CSU/DSU

4) Which of the following function of OSI Layer 2 is specified by the protocol standard for PPP, but is implemented with a Cisco Proprietary header field for HDLC?

E) Identifying the type of protocol that is inside the frame

5) Which of the following WAN data link protocols on Cisco routers support multiple Layer protocols by virtue of having some form of Protocol Type field?

A) PPP

B) HDLC

C) LAPB

6) On a point-to-point WAN link between two routers, what device(s) are considered to be the DTE devices?

A) Routers

7) Imagine the Router1 has three point-to-point serial links, one link each to three remote routes. Which of the following is true about the required DHLC addressing at Router1?

E) NONE

8) What is the name of the Frame Relay field used to identify Frame Relay Virtual Circuits?

A) Data-link connection identifier (DLCI)

9) Which of the following is true about Frame Relay virtual circuits?

B) Multiple VCs can share the same access link.

10) Which of the following defines a SONET link speed around 155 Mbps?E) OC-3

1) Are DLCI addressing defined by a Layer 2 for Layer 3 protocol?

DLCI addresses are defined by a Layer 2 protocol. Although they are not covered specifically in this chapter, Frame Relay protocols do not define a logical addressing structure that can usefully exist out side a Frame Relay network by definition, the addresses would be OSI Layer 2 equivalent.

2) What OSI layer typically encapsulates using both a header and a trailer?

The data link layer typically encapsulates using both a header and a trailer. The trailer typically includes a frame check sequence (FCS), which is used to perform error detection.

3) Define the terms DCE and DTE tin the context of the physical layer and a point-to-point serial link.

At the physical layer, DTE refers to the device that looks for clocking from the device on the other end of the cable on a link.

The DCE supplies that clocking.

Example: The computer is typically the DTE, and most modems or CSU/DSU is the DCE.

At the data link layer, both X.25 and Frame Relay define a logical DTE and DCE. In this case, the customer premises equipment (CPE) such as a router and a CSU/DSU is the logical DTE,

And the service provider equipment (the Frame Relay switch and the CSU/DSU) is the DCE.

4) Which layer or layers of OSI are most closely related to the function of Frame Relay? Why?

OSI Layer 1 and 2. Frame Relay refers to well-know physical layer specification. Frame Relay does define headers for delivery across the Frame Relay cloud, making it a Layer 2 protocol. Frame Relay does not include any routing or logical addressing specification, so it is not a Layer 3 protocol.

5) What is the name of the field that identifies, or addresses, a Frame Relay virtual circuit?

The Data-Link connection identifier (DLCI) is used to identify a VC

6) True or False: “A leased line between two router provides a constant amount for bandwidth- never more and never less”?

True. A lease line creates the cabling equivalent of having a cable between the two routers, with the speed (clock rate)defined by the telco. Even when the routers have no data to send, the full band width is available to be used.

7) True or False: “Frame relay VCs provide a consistent amount of band width between two devices, typically router – never more and never less?

False, The provider assigns a guaranteed bandwidth, or CIR, for VC, but the routers on either end of the VC can send more then the CIR of data. As long as the service provider has enough capacity to support it, the frames are forwarded over the VC.

8) Explain how may DS0 channels fit into a T1, and why the total does not add up to the purported speed of a T1, which is 1.544

Each DS0 channel runs at 64 kbps. With 24 in a T1, the T1 speed seemingly would be 24 * 64, or 1.536Mbps. T1 also includes 8bps for management, which, when added to the 1.536 Mbps total, gives you the full T1 rate -1.544

9) Define the term synchronous?

The imposition of time ordering on a bit stream. Practically, a device will try to use the same speed as another device on the other end of a serial link. By examining transitions between voltage states on the link, the device can notice slight variation in the speed on each end and can adjust its speed accordingly.

10) Imaging a drawing with two routes, each connected to an external CSU/DSU, which each is connect with a four-wire circuit, as seen in this chapter. Describe the role of the devices in relation to clocking and synchronization.

The routers receive clocking from their respective CSU/DSU. One of the two CSU/DSU is configured as the master. The other CSU/DSU, as the slave, adjusts its clock to match the speed of the master CSU/DSU.

11) Imaging a drawing with two router, each connected to an external CSU/DSU, which each is connect to with a four-wire circuit, as seen in this chapter. List the words behind the acronyms DTE and DCE, and describe which devices in this imagined network are DTE and which are DCE.

DTE stands for data terminal equipment and DCE stands for Data communication equipment. The routers are DTEs, and the CSU/DSUs are DCEs.

12) Imagine a drawing with two routers, which connect to a frame relay switch over a local access link. Describe which devices in this imagined network are Frame Relay DTEs and which are Frame Relay DCEs

The routers are DTEs, and the Frame Relay switches are DCEs

13) Do HDLC and PPP, as implemented by Cisco routers, support protocol type fields and error detection?

Both protocols support a protocol type field and an FCS field to perfume error detection. PPP has both fields based on the protocol specifications; Cisco added the protocol type field to the standard HDLC header.

14) Imaging a point to point leased ling between two routers, with PPP in use. What re the names of the protocols inside PPP that would be used on this link? What are their main functions?

The PPP Link Control Protocol (LCP) controls and managers the link.

The PPP IP Control Protocol (IPCP) also would be used because you need a CP fro each Layer 3 protocol. IPCP can assign IP address to devices on the other end if a link.

15) What are some of the main similarities between Frame Relay and ATM?

Both use an access link to access the service provider.

Both use the concept of virtual circuit between DTE devices.

And both allow multiple VCs to cross a single Access ling.

16) Compare and contrast ATM and SONET in term so the OSI model?

SONET defines the Layer details of passing traffic over optical cabling whereas ATM provides the Layer 2 functionality, including link-specific addressing, framing, and error detection.

17) Besides HDLC and PPP. List the other four serial point-to-point data-link protocols covered I this chapter?

SDLC

LAPB

LAPD

LAPF

18) List the speed of a T1 line, E1, OC-3 and OC-12

T1 line = 1,544

E1 = 2.048

OC-3 = 155Mbps

OC-12 = 622Mbps

1) Which of the following acronyms identifies a voice codec used to encode analog voice signals into a 64-Kbps digital stat stream?

D) PCM

2) How may DSO channels are in a DS1 in the United States?

E) 24

3) Which of the following best describes the function of demodulation by a modem?

A) Decoding an incoming analog electrical signal into a set of binary digits.

Modems demodulate an analog signal sent by the phone company. The goal is to re-create the original bits sent by the other modem. SO the demodulation function converts the analog signal into the bits that is was intended to represent.

4) Which of the following modem standards do not support 56-bps speeds downstream?

A) V.22

B) V.22bis

C) V.42

F) V.32

G) V.32bis

H) V.34

5) Which of the following terms best describes features of an ISDN PRI in Europe?

E) 30B+D

PRI’s in Europe are based on E1 circuits, which have 32 DS0 channels. One channel is reserved for framing, and one channel is used for a D channel, leaving 30 B channels.

6) Imagine that you plug an analog phone into an ISDN modem and call a friend at her house, where she is uses an analog phone using plain-old telephone service (POTS). At which the following points in a network will a codec be used?

B) The phone switch into which you friends’ local line is connects.

B) Your ISDN modem

Because the ISDN modem sends only digital signals over the local loop, it must convert the analog voice form the phone connected to it into digital voice using a voice codec.

7) What does the letter B stand for in the ISDN term B channel?

A) Bearer

8) which of the following DSL standards has a limit of 18,000 feet for the length of the local loop?

C) ADSL

9) Imagine a local phone line form a house to a local telco CO. When the customer at the house requests DSL service, what type of device does the telco move the CO end of the local line to?

A) DSLAM

The CO uses a DSLM to terminate local loop that use DSL. A DSL router or DSL modem is connected to the local loop at the subscriber (home) location.

10) Which of the following protocols are used by DSL modem and router for data link layer functions?

A) PPP

B) IEEE 802.3

C) ATM

DSL call for the use of PPP over ATM (PPoA) over the DSL part of the network. PPP can extend to the PCs at the home location using PPP

over Ethernet.

11) Which of the following protocols is used by cable modems for data link layer functions?

E) MCNS MAC

Multimedia Cable Network Services (MCNS) defines a MAC layer that also uses IEEE 802.2 as part of the data link layer.

12) Which of the following Protocols are used by a cable modem for the upstream data?

A) PCM

C) QAM-64

ISDN always uses symmetric speeds, and cable modems always use asymmetric speeds.

13) Which of the following remote access technologies uses ATM, Ethernet, ahd PPP as data-link protocols?

C) DSL

14) Which of the following remote access technologies support specifications that allow both symmetric and asymmetric speeds?

ISDN always use symmetric speeds, and cable modems always use asymmetric speeds.

15) Which of the following remote access technologies, when used to connect to an ISP, is considered to be an “always on” service?

C) DSL

D) CABLE MODEMS

Analog modems and ISDN lines must signal or dial to set up a circuit before any data can be passed, whereas DSL and cable modems do not do this.

a. What do ISDN, BRI and PRI stand for?

ISDN stands for Integrated Services Digital Network

BRI stands for Basic Rate Interface

PRI stands for Primary Rate Interface

b. How many bearer channels are in a BRI? What about a PRI in North America? What about a PRI in Europe?

BRI uses two bearer channels and one signaling channels (2B+D).

PRI uses 23+D in North America

30+D in Europe.

The signaling channel on BRI is a 16 kbps channel;

On PRI, it is a 64-kbps channel

c. Define what a voice codec does, and explain why a PCM codec needs 64 kbps for a single voice call.

Voice codec’s code and decode voice signals, converting form analog to digital, and digital to analog. A PCM codec samples the analog signal 8000 times per second, generation a 8-bit code to represent each sample. So, 64,000 bits are needed for a single second of voice.

d. Two terms were shortened and combined to first create the word modem. Identify these two words and describe what each word means.

The term modem is formed as a combination of the words modulation and

DEMODULATION.

MODULATION means to vary or change a wave form to encode information

A modem varies an analog electrical signal to encode information, representing binary digits, onto an analog signal. Modulation refers to the creation of the analog signal based on a string of bits, and demodulation simply refers to a modem performing the reverse process upon receiving the analog signal.

e. Define what the term symmetric and asymmetric mean in relation to modem specification. Also explain why asymmetric might be a better option.

Symmetric mean that the speed in each direction of flow is the same, whereas Asymmetric means that the speed in one direction is faster than the other. Asymmetric speed might be a good choice because typical traffic flows require a much greater amount of data to flow in one direction, typically form a server to a client. Asymmetric speeds allow the speed in one direction to be faster than it could be with symmetric speeds, accommodation the need for the more bandwidth in one direction.

2) Compare the V.90 and V.92 modem specification.

V.92 is an improvement over the V.90 standard.

V.92 supports symmetric and asymmetric speeds, whereas V.90 supports only asymmetric speeds,

V.90 supports only asymmetric speeds.

The upstream speed has been increased form 33 kbps up to 48kbps. It supports modem-on-hold, which allows the user to accept a voice call in response to a call-waiting signal, putting the modem connection on hold. It also senses the correct operational speed more quickly than V.90

3) Compare analog modems, ISDN BRIs, DSL, and cable modems in terms of concurrent support for voice and data.

Analog modems do not support concurrent voice and data transmissions.

ISDN and DSL both support simultaneous voice and data over the same local loop (local phone line) Cable allows simultaneous data, voice , and TV reception.

4) Compare analog modems, ISDN BRIs, DSL, and cable modems in terms of whether eth data device is always on.

Analog modems and ISDN BRIs Must signal to set up a circuit, so any data capabilities, such as Internet connectivity, are not “always on.” DSL and cable do not require any signaling to set up a circuit – fact, no circuit is needed in the PSTN to support these technologies, so these services are “are always on”

5) List some of the pros and cons regarding the use of analog modems for remote access.

Modems have the great advantage of being the most pervasively available remote access technology. The history of modems is long, with modems growing to be a very reliable choice for remote access. Speeds have improved over the years, with compressing technology increasing the effective throughput to beyond 100kbps. The biggest negatives about using modems include their relatively low speed and the fact that you cannot use the phone at the same time as you send data.

6) List some of the pros and cons regarding the use of ISDN for remote access.

ISDN’s advantages include the capability to support voice calls concurrently with a data call. Also, ISDN can be used over the local telco loop, with no significant distance limitation. And it provides more bandwidth than do modems, particularly with both B channels dialed to the same remote site. ISDN does have a few disadvantages, with the biggest disadvantage being the lower speeds than DSL, or Cable.

7) List some of the pros and cons regarding the use of DSL for remote access.

DSL provides high-speed Internet access to the home, exceeding downstream speeds of 1 Mbps. It supports concurrent voice and data, with the data service always being turned on- no dialing is required. And the service speed does not degrade when more user are added to the network. However, DSL simply will not be available to some people, based on the distance to the local CO or the availability of DSL , service form the local telco. Also, even when the home is close enough to the CO, sites farther form the CO might urn slower than sites closer to the CO.

8) Define what the acronym DSLM stands for, and explain the concept behind how a DSLAM allows voice and data to folw over the same local loop phone line.

DSLAM stands for DSL access multiplexer, with DSL meaning digital subscriber line. The DSLAM is connected to the local loop, splitting off the voice frequencies (0 – 4000 Hz) for the voice switch in the CO. It also interprets the higher frequencies as encoded digital signals, receiving the ATM cells sent over that digital signal, and forwards those ATM cells to the appropriate router.

9) Which of the DSL standards is the most common in the United States today? What is the range of upstream and downstream speeds for that type of DSL, as well as the maximum distance of the local loop?

ADSL, meaning asynchronous DSL, is the most popular, The downstream speeds range from 1.5 Mbps, whit upstream speeds form 64 to 800 kbps. The maximum distance is 18,000 feet (approximately 5500 meters).

10) What protocols are used by DSL at the data link layer?

ATM, ETHERNET, PPP

11) Imaging that Andy and Barney are neighbors, and they both use cable modems. Describe the type of traffic that they could generate that could cause collision, and tell what is done to help prevent those collisions.

Downstream data can never cause a collision with upstream data because the two are sent in different frequency ranges. Because only the head end sends downstream data, no collisions can occur. Upstream from all subscribers uses the same frequency range, so data sent to the head end by Andy and Barney could collide. Cable standards uses a feature called time division multiple access (TDMA) to assign time slots to each subscriber. This prevents most collisions to no collision should occur.

12) Name the four different Layer 1 encoding methods defined for use by cable modems. For each on , list whether it is used for upstream data, downstream data, or both.

QAM-64 and QAM-256 are both available for use as downstream encoding methods.

QAM-16 and QPSK are both available for upstream encoding.

13) Which of the four different remote access technologies support IP, TCP, UDP, and the rest of the higher-layer TCP/IP protocols?

ALL OF THEM

14) Compare and contrast the cabling used by an analog modem and a DSL router/modem when connecting to the local phone company line, Identify the purpose of each pin on the connector.

Both use a cable with two wires, using an RJ-11 connector. Pin 3 is used for transmit; Pin 4 is used for receive.

15) Compare and contrast the cabling used by an ISDN modem and a cable modem when connection to the local phone company line or cable drop line. Identify the purpose of each pin on the connector.

ISDN uses a four- wire cable using an RJ-45 connector. The Pinouts: pins 3 and 6 are for transmit, and pins 4 and 5 for receive. Cable modems use coaxial cable with a single conductor, so there are no pins. The round connector on the end of the cable is called an F-connector.

16) List four standards bodies that have been involved in the development of DSL standards

ANSI, IEEE, ETSI, ITU.

wide area networking

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