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
sho int s0/0 | inc LMILMI 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:
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 ?cisco ansi q933a
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 ...