Frame Relay

Semester 4, Chapter 6Allan Johnson

Table of Contents

Frame Relay Technology

Local Management Interface

The Use of Subinterfaces

Configuring Frame Relay

Go There!

Go There!

Go There!

Go There!

Frame Relay Technology

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Frame Relay History

Frame Relay was designed as a stream-lined version of X.25.

X.25, a 1970s ITU-T standard, ensured reliable transport at the data link layer with error detection and error correction.

With the introduction of DoD’s TCP/IP in the early 1980s, TCP took over error correction.

Although Frame Relay detects errors at the data link layer, it does not correct. That’s now TCP’s job.

Frame Relay History

In 1990, Cisco Systems, StrataCom, Nothern Telecom and Digital Equipment (sometimes referred to as the Group of Four) worked to standardize the Frame Relay protocol and add what they dubbed LMI extensions.

Today, Frame Relay is the most popular WAN protocol because it is: Faster than X.25, it uses TCP for error correction Cost-effective - you no longer have to pay for a

dedicated point-to-point link Versatile - can operate over a variety of interfaces

(ISDN, Serial, Dial-up, etc.)

Frame Relay Operation

Devices in the Frame Relay network are the DTE (customer equipment) and DCE (provider’s frame relay switch)

Often cheaper than other technologies because many times the service provider also owns the DTE.

The Frame Relay connection between the DTE and DCE operates at the data link and physical layers of the OSI model.

Data Link

Physical

Frame Relay Operation

Frame Relay operates over a permanent virtual circuit (PVC), which means that a permanent connection exists between the source DCE and destination DCE over the frame relay network.

Therefore, there is no need for call setup and termination like in ISDN. Frame Relay has two states: Data transfer--between the DCE and the provider’s

DTE Idle--the line is active, but no data is being

transferred.

Frame Relay Operation

The connection between the local DTE and the DCE in a Frame Relay network is logically identified with a Data-link Connection Identifier (DLCI).

A word about Switched Virtual Circuits (SVC) Frame Relay over ISDN must use a SVC with

ISDN’s call setup and termination procedures. However, currently few manufacturers of DCE

equipment support Frame Relay SVCs, so implementation is minimal.

Therefore, we will assume a PVC when discussing Frame Relay.

Frame Relay Lexicon

DLCI - Identifies logical connections to the Frame Relay network and has local significance only

FECN - Forward explicit congestion notification; tell receiving DTE to implement congestion avoidance procedures

BECN - Backwards explicit congestion notification; tells the sending DTE to slow down the transfer rate by 25%.

DE - Discard eligibility; bit set in the frame to say “frame is not business critical” and can be discarded

CIR - Committed information rate guaranteed by the service provider.

LMI - Local Management Interface; determines the operational status of PVCs

DLCI-identifies logical connections on the Frame Relay switch to which the customer is attached

BECN-tells sending DTE device to reduce the rate of sending data.

FECN-tells receiving DTE device to implement congestion avoidance procedures

FRAMES

BECN FECN

Frame Relay Frame Format

Flag - like most frame formats, the flag indicates the beginning and end of the frame

The DLCI makes up the first 10 bits of the address field, while the FECN, BECN, and DE bits are the last 3 bits.

Local Management Interface

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LMI Overview

LMI, similar to LCP in PPP, is a set of extensions to the basic Frame Relay protocol.

LMI’s main functions are to: determine the operational status of the PVC between source

and destination. transmit keepalives to ensure PVC stays up inform router what PVCs are available

LMI extensions were added by the Group of Four. A common extension that must be used by all who

implement Frame Relay is virtual circuit status messages. Optional extensions include...

• Multicasting• Global addressing• Simple Flow Control

LMI Frame Format

The LMI Frame has four mandatory bytes. They are outlined in red below. Know them!!

Frame Relay Map

The term map means to “map” or bind a Layer 2 address to a Layer 3 address. An ARP table maps MACs to IPs in a LAN In ISDN, we use the dailer-map command to map

SPIDs to IP addresses In Frame Relay, we need to map the data link

layer’s DLCI to the IP address We use the frame-relay map command

Frame Relay Map

The Frame Relay switch builds a table of incoming/outgoing ports and DLCIs.

The router builds a Frame Relay Map through Inverse ARP requests of the switch during the LMI exchange process.

The Frame Relay Map is used by the router for next-hop address resolution.

Use of Subinterfaces

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Using a Subinterface

In order to have an active Frame Relay link to all your routers in the network, you need either All routers on the same network or subnet or use subinterfaces

All Routers in Same Network

Frame Relay Network

192.168.4.0/24

192.168.4.1

192.168.4.3192.168.4.2

DLCI 17 DLCI 18

DLCI 16

A

B C

Using a Subinterface

Rarely do you have all routers on the same network or subnet, so we use subinterfaces.

Each serial interface can logically be divided into as many subinterfaces as you need to establish PVCs with each destination.

Each destination’s DLCI needs a separate point-to-point subinterface.

Each side of the PVC must belong to the same network.

A

B

C

D

Subinterface Example #1

S0.1 192.168.1.1

S0.2 192.168.2.1

S0.3 192.168.3.1

192.168.1.2

192.168.2.2

192.168.3.2

Each PVC as a point-to-point link in its own network

or subnet

Subinterface Example #2

Frame Relay Network

S0.16 192.168.4.2

DLCI 17 DLCI 18

DLCI 16

A

B CS0.18 192.168.6.1

S0.16 192.168.5.2

S0.17 192.168.6.2

S0.18 192.168.5.1S0.17 192.168.4.1

AB-PVC AC-PVC

BC-PVC

Configuring Frame Relay

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Frame Relay Encapsulation

To enable Frame Relay, simply go to the serial interface and enter the command

However, if you are connecting to a non-Cisco remote router, you must specify the option IETF

Router(config)#encap frame-relay [cisco|IETF]

Router(config)#int s0

Router(config-if)#encapsulation frame-relay

One Subnet/Network Configuration

The curriculum says to use a subinterface and specify multipoint.router(config-if)#int s1.1 multipoint

However, subinterfaces are not necessary when configuring PVCs on the same subnet.

Also, we do not have to set the LMI type since our Cisco IOS is 11.2 or later. LMI type is autosensed.

However, we enter a map command to link the remote rouer’s DLCI to its IP address.

One Subnet/Network Configuration

Configure frame relay encapsulation and map the destinations’ DLCIs to their IPs

Repeat the commands on each router in the network

RouterA(config)#int s0

RouterA(config-if)#encapsulation frame-relay

RouterA(config-if)#frame-relay map ip 192.168.4.2 17 broadcast

RouterA(config-if)#frame-relay map ip 192.168.4.3 18 broadcast

Frame Relay Network

192.168.4.0/24

192.168.4.1

192.168.4.3192.168.4.2

DLCI 17 DLCI 18

DLCI 16

A

B C

Multiple Subnet/Network Configuration

When your routers are parts of different networks or subnets, then you must either physically attach them on different interfaces or use subinterfaces on a single interface.

First step is to set the encapsulation type of the serial interface and state no IP address.

RouterA(config)#int s0RouterA(config-if)#encap frame-relayRouterA(config-if)#no ip addressRouterA(config-if)#no shut

Then enter subinterface configuration mode to assign each point-to-point link its IP address and define the destination’s DLCI.

Next slide shows the subinterface commands.

Multiple Subnet/Network Configuration

Frame Relay Network

S0.16 192.168.4.2

DLCI 17 DLCI 18

DLCI 16

A

B CS0.18 192.168.6.1

S0.16 192.168.5.2

S0.17 192.168.6.2

S0.18 192.168.5.1S0.17 192.168.4.1

AB-PVC AC-PVC

BC-PVC

RouterA(config-if)#int s0.17 point-to-point

RouterA(config-subif)#ip address 192.168.4.1 255.255.255.0

RouterA(config-subif)#frame-relay interface-dlci 17

RouterA(config-subif)#int s0.18 (continue with configuration)

Additional Frame Relay Labs

Chapter 6 does not provide enough practice for configuring Frame Relay.

If you have an Adtran unit, use the first two labs in Chapter 8 of the Semester 6 curriculum to gain additional Frame Relay experience. Lab 8.2.1: Configuring Frame Relay

• A “One Subnet/Network” Frame Relay Configuration Lab 8.3.4: Configuring Frame Relay with

Subinterfaces• A “Multiple Subnet/Network” Frame Relay Configuration

Verifying Frame Relay

show interface serial 0 is Frame Relay sending and receiving data? displays both LMI and DLCI information

show frame-relay map displays the frame relay table on the router

show frame-relay pvc used to verify a frame relay configuration

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