Frame Relay LMI

The Frame Relay LMI is a set of Frame Relay specifications. The original LMI was developed in 1990 by the Gang of Four (Cisco, DEC, Nortel and StrataCom). The LMI specifications include support for the following:

• Keepalive mechanism Verifies the data flow across the UNI (User-Network Interface)

• Multicast mechanism Provides the network server with local and multicast DLCI information

• Global addressing Gives DLCIs global rather than local significance

• Status mechanism Provides on-going status reports of each known DLCI on the network service provider's switch

Figure 10-8 illustrates the end-points for LMI Status Messages.

Figure 10-8. LMI Status Message End-points

The original LMI specification enables a number of enhancements to the original Frame Relay protocol for managing Frame Relay networks. The most noteworthy LMI extensions enable support for the following:

• Global addressing Giving Frame Relay DLCI values a global, rather than local, significance. These global DLCI values become Frame Relay networking device addresses that are unique in the Frame Relay WAN.

• Virtual circuit status messages Provide communication and synchronization between FRADs and the network service provider switches. These status messages report at regular intervals the up/down status of configured PVCs.

• Multicasting LMI extension supports the assignment to and management of multicast groups. Multicasting conserves bandwidth by enabling routing updates and address-resolution (for example, ARP, RARP) messages to be sent only to specific router groups.

There are three LMI types found in Frame Relay WAN implementations. These three types are listed here:

• ANSI T1.617, (Annex D) Maximum number of connections (PVCs) supported is limited to 976. LMI type ANSI T1.627 (Annex D) uses DLCI 0 to carry local (link) management information.

• ITU-T Q.933, (Annex A) Like LMI type Annex-D, the maximum number of connections (PVCs) supported is limited to 976. LMI type ITU-T Q.933 (Annex A) also uses DLCI 0 to carry local (link) management information.

• LMI (Original) Maximum number of connections (PVCs) supported is limited to 992. LMI type LMI uses DLCI 1023 to carry local (link) management information.

NOTE

The LMI (Original) type is annotated as LMI type "Cisco" within the Cisco IOS.