Institute of Technology, Sligo Dept of Computing Frame Relay Technology Semester 4 Chapter 6

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.

Institute of Technology, Sligo Dept of Computing Local Management Interface

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 yellow below.

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.

Institute of Technology, Sligo Dept of Computing Use of Subinterfaces

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 / 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 S S S Each PVC as a point-to-point link in its own network or subnet

Subinterface Example #2 Frame Relay Network S DLCI 17 DLCI 18 DLCI 16 A B C S S S S S AB-PVCAC-PVC BC-PVC

Institute of Technology, Sligo Dept of Computing Configuring Frame Relay

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 broadcast RouterA(config-if)#frame-relay map ip broadcast Frame Relay Network / 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 s0 RouterA(config-if)#encap frame-relay RouterA(config-if)#no ip address RouterA(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 S DLCI 17 DLCI 18 DLCI 16 A B C S S S S S AB-PVCAC-PVC BC-PVC RouterA(config-if)#int s0.17 point-to-point RouterA(config-subif)#ip address RouterA(config-subif)#frame-relay interface-dlci 17 RouterA(config-subif)#int s0.18 (continue with 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