Enterasys Networks Security Router X-PeditionTM - Manual (Page 209)

XSR User’s Guide 9-1
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 9-1 on page 9-2 for 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-relay interface-dlci command maps a DLCI to a specified FR sub-interface.