8.1 Frame Relay
8.1.3 Frame Relay signaling
LMI is a signaling standard between the CPE device and the Frame Relay switch that is responsible for managing the connection and maintaining status between the devices. As shown in the Figure, LMIs include support for the following:
  • A keepalive mechanism, which verifies that data is flowing
  • A multicast mechanism, which provides the network server with its local DLCI
  • The multicast addressing, which gives DLCIs global rather than local significance in Frame Relay networks
  • A status mechanism, which provides an ongoing status on the DLCIs known to the switch

Although the LMI is configurable, beginning in Cisco IOS Release 11.2, the router tries to autosense which LMI type the Frame Relay switch is using by sending one or more full status requests to the Frame Relay switch. The Frame Relay switch responds with one or more LMI types. The router configures itself with the last LMI type received. Three types of LMIs are supported:

  • cisco LMI - type, defined jointly by Cisco, StrataCom, Northern Telecom, and Digital Equipment Corporation, nicknamed “the gang of four”
  • ansi Annex - D of the ANSI standard T1.617
  • q933a ITU-T Q.933 Annex A

An administrator setting up a connection to a Frame Relay network must choose the appropriate LMI from the three supported types to ensure proper Frame Relay operation.

Frame Relay Extensions

In addition to the basic Frame Relay protocol functions for transferring data, the "gang of four" promoted the following extensions:

Virtual circuit status messages (common)-These messages provide communication and synchronization between the network and the user device. They periodically report the existence of new PVCs and the deletion of already existing PVCs, and generally provide information about PVC integrity. VC status messages prevent the sending of data into black holes, that is, over PVCs that no longer exist.

Multicasting (optional)-Multicasting 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 procedures that typically must be sent to many destinations simultaneously.

Global addressing (optional) -Global addressing gives connection identifiers global rather than local significance, which allows 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. Therefore, address resolution protocols perform over Frame Relay exactly as they do over a LAN.

Simple flow control (optional)-This provides for an XON/XOFF flow-control mechanism that applies to the entire Frame Relay interface. It is intended for those devices whose higher layers cannot use the congestion notification bits and that need some level of flow control.

When an inverse ARP request is made, the router updates its map table with three possible LMI connection states, as follows:

  • Active state - indicates that the connection is active and that routers can exchange data
  • Inactive state - indicates that the local connection to the Frame Relay switch is working, but the remote router connection to the Frame Relay switch is not working
  • Deleted state - indicates that no LMI is being received from the Frame Relay switch, or that there is no service between the CPE router and Frame Relay switch