Jan 15 2009
Frame-Relay LMI
What is LMI?
LMI stands for Local Management Interface.
LMI messages are exchanged between routers(DTE) and frame-relay(DCE) switches. At its very basic we can say the LMI messages act as keepalives between the router and frame-relay switch. LMI messages also contain information which allows routers to detect new PVCs dynamically. Cisco IOS currently supports 3 types of LMI Cisco, ANSI Annex D and Q.933-A. In order for LMI to work you must use the same LMI type on both the router and frame-relay switch.
All three LMI types listen for LMI messages on reserverd DLCIs.
ANSI and q.933 listen for messages on DLCI 0 and Cisco LMI listens for messages on DLCI 1023.
Consider the topology below.
As you can see the LMI needs to match between the local DTE and the DCE ie they need to be speaking the same language.
Cisco IOS actually supports an LMI autosense feature which allows a router(DTE) to dynamically detect the LMI type configured on the connected frame-relay switch.
Also worth noting on the diagram. I preconfigured a PVC on the frame-relay switch with a DLCI if 102 facing R1. From the debug information you can see that this DLCI is passed to R1.