The Local Management Interface (LMI) is a set of extensions to the Frame Relay protocol that were designed to provide information about the status of Frame Relay networks, and extend the technology’s capabilities. LMI is primarily concerned with diagnostic functions. For example, LMI is used to send keepalive messages between a router and a Frame Relay switch. These messages verify the status of PVCs, and help to ensure that data can be transferred between Frame Relay DTE and DCE equipment. The LMI extensions also allow DLCI values to be made globally significant, and bring multicast capabilities to Frame Relay networks.
Both ANSI and the ITU-T have also developed LMI standards. As of IOS version 11.2, a Cisco router interface using Frame Relay encapsulation will auto-sense the LMI type used by a Frame Relay switch. However, it is still possible to configure the LMI type manually. The three different LMI types supported on a Frame Relay interface include:
- Cisco. This is the “Group of Four” LMI type, and is the default used on Frame Relay interfaces prior to IOS version 11.2. It uses DLCI 1023 to exchange status information.
- ANSI. This is the LMI type defined by ANSI. It uses DLCI 0 to exchange status information.
- Q.933a. This is the LMI type defined by the ITU-T. It uses DLCI 0 to exchange status information.
A router and its local Frame Relay switch must be using the same LMI type in order for diagnostic messages to be passed between them.