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Frame Relay

Frame Relay. CCNA4 Chapter 6. Frame Relay Overview. Layer 2 Connection-oriented Packet switched Virtual Circuits: Permanent (and Switched) Links two DTE devices through DCE Originally designed for operation over ISDN Faster than X.25 Encapsulation can be Cisco or IETF.

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Frame Relay

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  1. Frame Relay CCNA4 Chapter 6

  2. Frame Relay Overview • Layer 2 • Connection-oriented • Packet switched • Virtual Circuits: Permanent (and Switched) • Links two DTE devices through DCE • Originally designed for operation over ISDN • Faster than X.25 • Encapsulation can be Cisco or IETF RouterA(config-if)#encapsulation frame-relay ? ietf Use RFC1490 encapsulation

  3. Frame Relay Operation

  4. Frame Relay - DLCI • Data Link Connection Identifier • IP addresses must be resolved to DLCI • Logical Virtual Circuits are distinguished by DLCI • Only locally significant • DLCI do not need to match end to end, though the may do if connected to different switches • DLCI are assigned by the service provider. • FR switch maps DLCI numbers to outgoing ports

  5. DLCI C DLCI=100 E DLCI=101 DLCI=102 A DLCI=16 B DLCI=100 D DLCI=100 DLCI=101 DLCI=201

  6. DLCI Mapping • The FR switch uses DLCI numbers to communicate • Network layer addresses must be mapped Static mapping: RouterA(config-if)#frame-relay map ip 172.16.30.19 18 Dynamic mapping uses Inverse ARP: • Router learns DLCI from switch during initial LMI exchange • Router sends IARP request for each DLCI • 3. The return information is then used to build the map DLCI = 100 What’s the next hop address for DLCI 100? My IP address:172.16.1.2

  7. Frame Relay Rates Network performance at the router level is affected by: Access Rate Line speed of connection to the provider CIR Guaranteed bandwidth Bursting Faster than CIR for a short time period (Tc) Committed Rate measurement interval (Bc) Committed Burst (Be) Excess Burst – maximum transfer attempted DE = Discard Eligibility bit. Set on oversubscribed traffic

  8. LMI -Local Management Interface • Signalling between router and frame switch • Determine which PVCs are available and their status • LMI DLCI=1023 • Extensions to basic frame relay protocol: Common: Status of virtual circuits (Keepalives) Optional: Multicasting (DLCI 1019-1022) Global Addressing Simple Flow Control • Cisco IOS 11.2 and later uses auto-detection • Cisco routers support three versions of LMI: RouterA(config-if)#frame-relay lmi-type ? cisco ansi q933a

  9. Frames Frame Relay Format LMI Frame Mandatory bytes 1023 Status-enquiry message (from user device) Status message:keepalives and PVC status

  10. Frame Relay Subinterfaces FR can divide a single physical WAN interface into multiple subinterfaces. Subinterfaces are logical subdivisions of the physical interface. Split-Horizon issues can be resolved. RouterA(config)#int s0 RouterA(config-if)#encapsulation frame-relay RouterA(config-if)#int s0.1 ? multipoint Treat as a multipoint link point-to-point Treat as a point-to-point link Point to Point is used when each PVC is a separate subnet. Multipoint is used when the PVCs use the same subnet.

  11. Frame Relay Subinterfaces Configure the local DLCI for the subinterface to distinguish it from the physical interface: RouterA(config-if)#frame-relay interface-dlci ? <16-1007> Define a DLCI as part of the current subinterface RouterA(config-if)#frame-relay interface-dlci 16 Example configuration: RouterA(config)#interface Serial0 RouterA(config-if)# no ip address RouterA(config-if)# encapsulation frame-relay RouterA(config-if)# interface Serial0.102 point-to-point RouterA(config-if)# ip address 10.1.12.1 255.255.255.0 RouterA(config-if)# frame-relay interface-dlci 102

  12. Verifying Frame Relay show interface s0 show frame-relay pvc show frame-relay map show frame-relay lmi

  13. Questions Q- What is the first step in configuring Frame-Relay on an interface? A- Setting the encapsulation Q- What are the possible types of LMI? A- Cisco, ANSI and Q.933a Q- What is the solution to the issue of split-horizon when configuring multiple VCs on a single physical interface? A- Sub-interfaces Q- What command can you issue to see your PVCs? A- show frame-relay pvc Q- What three bits does frame-relay use for congestion control? A- FECN, BECN, DE Q- What DLCI value defines an LMI frame? A- 1023

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