Institute of Technology,Sligo Dept of Computing

Frame Relay TechnologyFrame Relay Technology

Semester 4 Semester 4

Chapter 6Chapter 6

Frame Relay HistoryFrame Relay History Frame Relay was designed as a stream-lined Frame Relay was designed as a stream-lined

version of X.25.version of X.25. X.25, a 1970s ITU-T standard, ensured reliable X.25, a 1970s ITU-T standard, ensured reliable

transport at the data link layer with error transport at the data link layer with error detection detection andand error correction. error correction.

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

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

Frame Relay HistoryFrame Relay History In 1990, Cisco Systems, StrataCom, Nothern Telecom In 1990, Cisco Systems, StrataCom, Nothern Telecom

and Digital Equipment (sometimes referred to as the and Digital Equipment (sometimes referred to as the Group of Four) worked to standardize the Frame Relay Group of Four) worked to standardize the Frame Relay protocol and add what they dubbed LMI extensions.protocol and add what they dubbed LMI extensions.

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

point-to-point linkpoint-to-point link VersatileVersatile - can operate over a variety of interfaces (ISDN, - can operate over a variety of interfaces (ISDN,

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

Frame Relay OperationFrame Relay Operation Devices in the Frame Relay network are the DTE (customer Devices in the Frame Relay network are the DTE (customer

equipment) and DCE (provider’s frame relay switch)equipment) and DCE (provider’s frame relay switch) Often cheaper than other technologies because many times Often cheaper than other technologies because many times

the service provider also owns the DTE.the service provider also owns the DTE. The Frame Relay connection between the DTE and DCE The Frame Relay connection between the DTE and DCE

operates at the data link and physical layersoperates at the data link and physical layers of the OSI of the OSI model.model.

Data Link

Physical

Frame Relay OperationFrame Relay Operation Frame Relay operates over a permanent virtual Frame Relay operates over a permanent virtual

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

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

DTEDTE IdleIdle--the line is active, but no data is being transferred.--the line is active, but no data is being transferred.

Frame Relay OperationFrame Relay Operation The connection between the local DTE and the The connection between the local DTE and the

DCE in a Frame Relay network is DCE in a Frame Relay network is logically identifiedlogically identified with a Data-link Connection Identifier (DLCI).with a Data-link Connection Identifier (DLCI).

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

call setup and termination procedures.call setup and termination procedures. However, currently few manufacturers of DCE equipment However, currently few manufacturers of DCE equipment

support Frame Relay SVCs, so implementation is support Frame Relay SVCs, so implementation is minimal.minimal.

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

Frame Relay LexiconFrame Relay Lexicon DLCIDLCI - Identifies logical connections to the Frame Relay - Identifies logical connections to the Frame Relay

network and has network and has local significance onlylocal significance only FECNFECN - Forward explicit congestion notification; tell receiving - Forward explicit congestion notification; tell receiving

DTE to implement congestion avoidance proceduresDTE to implement congestion avoidance procedures BECNBECN - Backwards explicit congestion notification; tells the - Backwards explicit congestion notification; tells the

sending DTE to slow down the transfer rate by 25%.sending DTE to slow down the transfer rate by 25%. DEDE - Discard eligibility; bit set in the frame to say “frame is - Discard eligibility; bit set in the frame to say “frame is notnot

business critical” and can be discardedbusiness critical” and can be discarded CIRCIR - Committed information rate - Committed information rate guaranteed by the service guaranteed by the service

providerprovider.. LMILMI - Local Management Interface; - Local Management Interface; determines the determines the

operational status of PVCsoperational 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 FormatFrame Relay Frame Format

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

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

Institute of Technology,Sligo Dept of Computing

Local Management InterfaceLocal Management Interface

LMI OverviewLMI Overview LMI, similar to LCP in PPP, is a set of extensions to LMI, similar to LCP in PPP, is a set of extensions to

the basic Frame Relay protocol.the basic Frame Relay protocol. LMI’s main functions are to:LMI’s main functions are to:

determine the determine the operational status of the PVCoperational status of the PVC between between source and destination.source and destination.

transmit keepalives to ensure PVC stays uptransmit keepalives to ensure PVC stays up inform router what PVCs are availableinform router what PVCs are available

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

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

MulticastingMulticasting Global addressingGlobal addressing Simple Flow ControlSimple Flow Control

LMI Frame FormatLMI Frame Format

The LMI Frame has four mandatory bytes. The LMI Frame has four mandatory bytes. They are outlined in yellow below. They are outlined in yellow below.

Frame Relay MapFrame Relay Map

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

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

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

Frame Relay MapFrame Relay Map The Frame Relay switch builds a The Frame Relay switch builds a table of table of

incoming/outgoing ports and DLCIsincoming/outgoing ports and DLCIs.. The router The router builds a Frame Relay Map through Inverse ARP builds a Frame Relay Map through Inverse ARP

requestsrequests of the switch during the LMI exchange process. of the switch during the LMI exchange process. The Frame Relay Map is used by the router for next-hop The Frame Relay Map is used by the router for next-hop

address resolutionaddress resolution..

Institute of Technology,Sligo Dept of Computing

Use of SubinterfacesUse of Subinterfaces

Using a SubinterfaceUsing a Subinterface In order to have an active Frame Relay link to all In order to have an active Frame Relay link to all

your routers in the network, you need eitheryour routers in the network, you need either All routers on the same network or subnetAll routers on the same network or subnet or use subinterfacesor 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 SubinterfaceUsing a Subinterface Rarely do you have all routers on the same Rarely do you have all routers on the same

network or subnet, so we use subinterfaces.network or subnet, so we use subinterfaces. Each serial interface can logically be divided into Each serial interface can logically be divided into

as many subinterfaces as you need to establish as many subinterfaces as you need to establish PVCs with each destination.PVCs with each destination.

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

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

A

B

C

D

Subinterface Example #1Subinterface 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 #2Subinterface 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

Institute of Technology,Sligo Dept of Computing

Configuring Frame RelayConfiguring Frame Relay

Frame Relay EncapsulationFrame Relay Encapsulation To enable Frame Relay, simply go to the serial interface and To enable Frame Relay, simply go to the serial interface and

enter the commandenter the command However, if you are connecting to a non-Cisco remote router, However, if you are connecting to a non-Cisco remote router,

you must specify the option you must specify the option IETFIETF

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

Router(config)#int s0

Router(config-if)#encapsulation frame-relay

One Subnet/Network ConfigurationOne Subnet/Network Configuration The curriculum says to use a subinterface and The curriculum says to use a subinterface and

specify multipoint.specify multipoint.router(config-if)#int s1.1 multipointrouter(config-if)#int s1.1 multipoint

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

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

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

One Subnet/Network ConfigurationOne Subnet/Network Configuration Configure frame relay Configure frame relay

encapsulation and map encapsulation and map the destinations’ DLCIs the destinations’ DLCIs to their IPsto their IPs

Repeat the commands Repeat the commands on each router in the on each router in the networknetwork

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 ConfigurationMultiple Subnet/Network Configuration When your routers are parts of different networks or subnets, When your routers are parts of different networks or subnets,

then you must either physically attach them on different then you must either physically attach them on different interfaces or use subinterfaces on a single interface.interfaces or use subinterfaces on a single interface.

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

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

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

Next slide shows the subinterface commands.Next slide shows the subinterface commands.

Multiple Subnet/Network ConfigurationMultiple 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)

Verifying Frame RelayVerifying Frame Relay

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

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

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