Enterasys Networks X-PeditionTM manual Virtual Circuits, DLCIs

9

Configuring Frame Relay

Overview

Frame Relay (FR) is a simple, bit-oriented protocol that offers fast-packet switching for wide-area networking. It combines the statistical multiplexing and port-sharing features of an X.25 connection with fast speed and low delay for high performance and less overhead. Frame Relay organizes data into variable-length, individually addressed units known as frames rather than placing them in fixed time slots for delivery over a packet-switched network where the data channel is occupied only for the duration of the transmission.

Virtual Circuits

FR is based on the Virtual Circuit (VC) - a two-way, always on, software-defined data path between two ports that acts as a “private” line in the network. The XSR supports Permanent Virtual Circuits (PVCs), multiplexing several PVCs in a single FR port, which reduces the number of physical connections required to link sites. FR can be configured with multiple PVCs connecting to remote sites. See Figure 9-1on page 9-2for a typical network topology.

DLCIs

The Data Link Connection Identifier (DLCI) is a unique number assigned to a PVC end point, essentially, the port to which the destination network is attached, as shown in Figure 9-1. DLCIs can perform data “interleaving” from two or more devices on a single channel known as statistical multiplexing. Data entering a FR switch are processed by the DLCI in three ways: frames are checked for integrity, their associated DLCI is looked up in the DLCI table, and they are relayed to their destination through the port specified in the table. If the checks reveal errors or do not find the DLCI in the table, frames are discarded.

The frame-relayinterface-dlcicommand maps a DLCI to a specified FR sub-interface.

XSR User’s Guide 9-1