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Network Warrior, 2nd Edition by Gary A. Donahue

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Local Management Interface

In 1990, Cisco Systems, StrataCom, Northern Telecom, and Digital Equipment Corporation developed a set of enhancements to the Frame Relay protocol called the Local Management Interface (LMI). LMI provides communication between the data terminal equipment, or DTE devices (routers, in our examples), and the data communication equipment, or DCE devices (telecom switches, in our examples). One of the most useful enhancements in LMI is the exchange of status messages regarding VCs. These messages tell routers when a Frame Relay PVC is available.

LMI messages are sent on a predefined PVC. You can see the LMI type and PVC in use with the show interface command:

Router-A#sho int s0/0 | inc LMI
  LMI enq sent  85, LMI stat recvd 86, LMI upd recvd 0, DTE LMI up
  LMI enq recvd 0, LMI stat sent  0, LMI upd sent  0
  LMI DLCI 1023  LMI type is CISCO  frame relay DTE

Three forms of LMI are configurable on Cisco routers: cisco, ansi, and q933a (Annex A). The DCE device (telecom switch) usually determines the type of LMI. The default LMI type on Cisco routers is cisco. You can change the LMI type with the frame-relay lmi-type interface command:

Router-A(config-if)#frame-relay lmi-type ?
  cisco
  ansi
  q933a

Congestion Avoidance in Frame Relay

Frame Relay includes provisions for congestion avoidance. Included in the Frame Relay header are two bits titled Forward-Explicit Congestion Notification (FECN, pronounced FECK-en), and Backward-Explicit Congestion Notification (BECN, pronounced BECK-en ...

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