CCNA Practical Studies (Cisco Certification & Training)

Frame Relay switches are very complex and complicated devices used by telecom services providers to provide cheap, reliable access to the service provider's network. The Frame Relay switch is the device that Cisco routers communicate with to negotiate packet transmissions. There are several important aspects to understanding how a Frame Relay switch operates.

First, you need to review two of the components of Frame Relay that involve the switch. Although there are several more, you need not be well versed in these for the CCNA exam. This appendix restricts definition of these terms in the perspective of the Frame Relay switch. The two components in question are as follows :

• Data-Link Control Identifier (DLCI) The identifying number of the PVC on the Frame Relay switch. This has local significance only.

• Local Management Interface (LMI) The Frame Relay Switch responds to LMI requests sent by a Frame Relay DTE device. This is the mechanism by which the Frame Relay switch announces to the Frame Relay DTE device (usually a router) the DLCI that the Frame Relay switch has been configured to propagate. This DLCI is only locally significant, meaning that the destination PVC does not need to have the same DLCI number.

A Frame Relay switch announces on the appropriate port the correct DLCI to a Frame Relay DTE device (router). Because Frame Relay is a Layer 1 and Layer 2 technology, Layer 3 protocols can be transported independently. The Frame Relay switch provides the physical layer path and the format specification for the Layer 2 frame.

Figure B-6 reviews the sequence of events that transpires between a Frame Relay switch and a Frame Relay DTE device.

As you can see in Figure B-6, the router sends an LMI keepalive to the Frame Relay switch, and then the Frame Relay switch responds to the keepalives and sends the appropriate DLCI information to the router.