Frame Relay Local Management Interface (LMI) is a set of enchantments to frame relay, originally agreed upon in 1990 by a consortium consisting of Cisco, Digital Equipment Corporation (DEC), Northern Telecom, and StrataCom. Eventually ANSI and CCITT came along created standard versions but most vendors support both standard version alongside the one defined by the consortium.
There are very few differences between them, mainly Cisco LMI type use the DLCI of 1023 and also DLCI number between 16 and 1007 for usable DLCI number and the ANSI & ITU(q933a) standard use a DLCI of 0 and usable DLCI numbers between 16 and 976. The LMI type can be changed on the frame relay enabled interface with the command;
Router(config-if)#frame-relay lmi-type ? cisco ansi q933a
The LMI messages messages sent between Routers and Frame Relay switches provide the the following additional features;
- Inverse ARP – to find out the layer 3 address of device on the other end of the DLCI so you dont need to static map it or similar
- Signalling of Virtual Circuit status messages, therefore if a PVC becomes unreachable all nodes along its path can be aware of this failure so data can be prevented from being sent to indirect failures.
- Multicasting – This extension allows multicast groups to be used over frame relay networks, the higher DLCI numbers reserved by the LMI type are used for this
- Globally signficiant DLCIs – Brilliant!
- Retro flow control with XON/XOFF if the applications using the Frame Relay network know understand FECNs and BECNs
By default LMI messages are sent every 10 seconds, and every sixth message a full status message will be sent which contains more detailed information about each VC, the interface will fail if the interfaces does not receive an LMI message 3 times the hello time, so 30 seconds. You cant actually turn off LMI but you can disable the keepalives with the highly ambiguous command;
Router(config-if)#frame-relay lmi-n391dte ? <1-255> event
Finally you can check the status of LMI with the command;
Router#sh frame lmi LMI Statistics for interface Serial1/0 (Frame Relay DTE) LMI TYPE = CISCO Invalid Unnumbered info 0 Invalid Prot Disc 0 Invalid dummy Call Ref 0 Invalid Msg Type 0 Invalid Status Message 0 Invalid Lock Shift 0 Invalid Information ID 0 Invalid Report IE Len 0 Invalid Report Request 0 Invalid Keep IE Len 0 Num Status Enq. Sent 18 Num Status msgs Rcvd 0 Num Update Status Rcvd 0 Num Status Timeouts 18 Last Full Status Req 00:00:03 Last Full Status Rcvd never
Finally pretty much every command on the interface beings with frame-relay;
Router(config-if)#frame-relay ? accounting Special accounting instruction address-reg ELMI address registration broadcast-queue Define a broadcast queue and transmit rate class Define a map class on the interface congestion-management Enable Frame Relay congestion management de-group Associate a DE group with a DLCI fragment Enable end-to-end fragmentation for all PVCs fragmentation Adaptive fragmentation
UPDATE: On some versions of IOS it forces you to set set a LMI hello of between 1 and 255 thereby the above method of using frame relay encapsulation on a P2P link by turning off LMI does not work, in these cases your going to have to use a different encapsulation type or a frame relay switch.
R5(config-if)#frame-relay lmi-n391dte % Incomplete command.