Frame Relay Technology

1. Frame Relay Technology Semester 4 Chapter 6

2. 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.

3. 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.)

4. Data Link Physical 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.

5. 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.

6. 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.

7. 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

8. FECN-tells receiving DTE device to implement congestion avoidance procedures FECN BECN FRAMES 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.

9. 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.

10. Local Management Interface

11. 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

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

13. 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

14. 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.

15. Use of Subinterfaces

16. A 192.168.4.1 DLCI 16 Frame Relay Network 192.168.4.0/24 192.168.4.2 192.168.4.3 DLCI 17 DLCI 18 C B 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

17. 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.

18. B A C S0.1 192.168.1.1 S0.2 192.168.2.1 S0.3 192.168.3.1 D Subinterface Example #1 Each PVC as a point-to-point link in its own network or subnet 192.168.1.2 192.168.2.2 192.168.3.2

19. A S0.17 192.168.4.1 S0.18 192.168.5.1 DLCI 16 AB-PVC AC-PVC Frame Relay Network S0.16 192.168.5.2 S0.16 192.168.4.2 DLCI 17 DLCI 18 C B BC-PVC S0.17 192.168.6.2 S0.18 192.168.6.1 Subinterface Example #2

20. Configuring Frame Relay

21. 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

22. 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.

23. A 192.168.4.1 DLCI 16 Frame Relay Network 192.168.4.0/24 192.168.4.2 192.168.4.3 DLCI 17 DLCI 18 C B 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

24. 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.

25. A S0.17 192.168.4.1 S0.18 192.168.5.1 DLCI 16 AB-PVC AC-PVC Frame Relay Network S0.16 192.168.5.2 S0.16 192.168.4.2 DLCI 17 DLCI 18 C B BC-PVC S0.17 192.168.6.2 S0.18 192.168.6.1 Multiple Subnet/Network Configuration 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)

26. 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