|  | 6.2 | LMI: Cisco's Implementation of Frame Relay
| 6.2.1 | LMI o peration
There was a major development in Frame Relay's history in 1990, when Cisco Systems, StrataCom, Northern Telecom, and Digital Equipment Corporation formed a group to focus on Frame Relay technology development and accelerate the introduction of interoperable Frame Relay products. This group developed a specification conforming to the basic Frame Relay protocol, but extended it with features that provide additional capabilities for complex internetworking environments. These Frame Relay extensions are referred to as LMI (local management interface). The main functions of LMI process are: To determine the operational status of the various PVCs that the router knows about To transmit keepalive packets to ensure that the PVC stays up and does not shut down due to inactivity To tell the router what PVCs are available Three LMI types can be invoked by the router: ansi, cisco, and q933a. LMI extensions In addition to the basic Frame Relay protocol functions for transferring data, the Frame Relay specification includes LMI extensions that make supporting large, complex internetworks easier. Some LMI extensions are referred to as common and are expected to be implemented by everyone who adopts the specification. Other LMI functions are referred to as optional. A summary of the LMI extensions follows: Virtual circuit status messages (common) -- Provide communication and synchronization between the network and the user device, periodically reporting the existence of new PVCs and the deletion of already existing PVCs, and providing general information about PVC integrity. Virtual circuit status messages prevent the sending of data over PVCs that no longer exist. Multicasting (optional) -- Allows a sender to transmit a single frame but have it delivered by the network to multiple recipients. Thus, multicasting supports the efficient conveyance of routing protocol messages and address resolution protocols that typically must be sent to many destinations simultaneously. Global addressing (optional) -- 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 local-area network (LAN) in terms of addressing; address resolution protocols therefore perform over Frame Relay exactly as they do over a LAN. Simple flow control (optional) -- Provides for an XON/XOFF flow control mechanism that applies to the entire Frame Relay interface. It is intended for devices whose higher layers cannot use the congestion notification bits and that need some level of flow control.