May 2011
Intermediate to advanced
788 pages
23h 34m
English
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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 DTEThree 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
q933aCongestion 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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