LOCAL MANAGEMENT INTERFACE (LMI)

LOCAL MANAGEMENT INTERFACE (LMI)

LOCAL MANAGEMENT INTERFACE

Local management interface(LMI) is a keepalive mechanism that provides status information about Frame Relay connections between the router (DTE) and the Frame Relay switch (DCE). Every 10 seconds or so, the end device polls the network, either requesting a dumb sequenced response or channel status information. If the network does not respond with the requested information, the user device may consider the connection to be down. When the network responds with a FULL STATUS response, it includes status information about DLCIs that are allocated to that line. The end device can use this information to determine whether the logical connections are able to pass data

LMI Extensions

The Frame Relay specification includes optional LMI extensions that are extremely useful in an internetworking environment. Some of the extensions include:

• Keepalives-These verify that data is flowing.

• Multicasting - Allows a sender to transmit a single frame that is delivered to multiple recipients. Multicasting supports the efficient delivery of routing protocol messages and address resolution procedures that are typically sent to many destinations simultaneously.

• Global addressing - Gives connection identifiers global rather than local significance, allowing them to be used to identify a specific interface to the Frame Relay network. Global addressing makes the Frame Relay network resemble a LAN in terms of addressing, and ARPs perform exactly as they do over a LAN.

• Status of Virtual Circuit - Provide information about PVC integrity by communicating and synchronizing between devices, periodically reporting the existence of new PVCs and the deletion of already existing PVCs. VC status messages prevent data from being sent into black holes (PVCs that no longer exist).

LMI Types

Three types of LMIs are supported by Cisco routers:

• Cisco – Original LMI extension
• Ansi - Corresponding to the ANSI standard T1.617 Annex D
• q933a – Corresponding to the ITU standard Q933 Annex A

Virtual Circuit Status

When an Inverse ARP request is made, the router updates its map table with three possible LMI connection states. These states are active state, inactive state, and deleted state

• ACTIVE State- indicates a successful end-to-end (DTE to DTE) circuit.
• INACTIVE State – indicates a successful connection to the switch (DTE to DCE) without a DTE detected on the other end of the PVC. This can occur due to residual or incorrect configuration on the switch.
• DELETED State – indicates that the DTE is configured for a DLCI the switch does not recognize as valid for that interface.