Short Bytes: Local Management Interface is a Cisco-proprietary technology which helps in managing various Permanent Virtual Circuits at a time using LMI extensions. It uses address resolution, keepalive packets and status messages for the same.
What is LMI?
Local Management Interface (LMI), a technology developed by Cisco, is a signaling standard that is used between the routers and frame relay switches for various purposes such as exchanging information keepalives timers, global addressing, multicasting and the current status of virtual circuits.
The local management interface (LMI) follows a global addressing which is responsible for giving an identifier number to a data link connection known as Frame Relay data-link connection identifier (DLCI).
For more clarity understand it this way:
Frame relay is like a direct connection between two endpoints on the internet. The end points are called as DTE (Data Terminal Equipment) and the device which helps the packet reach their destination is called DCE (Data Circuit-terminating equipment). It could be a modem as well.
So, when a dedicated connection is established between two points, the connection carries a unique identification number called DLCI. Because the DTE addresses are unique in the frame relay, they become the DLCI.
However, to provide a synchronization between the DTE and the DCE devices, LMI keeps on sending status messages periodically about the status of the permanent virtual circuit.
If there are no status messages and PVC has been taken down, the data is stopped from being sent out on the virtual circuit.
However, if there are multiple connections, then instead of going for individual message exchange, the LMI helps in multicasting. The multicasting allows multicast groups to be assigned and it also saves bandwidth by allowing routing updates and address resolution messages to be sent only to specific groups of routers.
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