User`s guide

XSR User’s Guide 123
Chapter 7 Controlling Congestion in Frame Relay Networks
Configuring Frame Relay
Controlling Congestion in Frame Relay Networks
While Frame Relay provides dedicated, logical channels throughout the
network, these channels share physical resources - links and Frame Relay
switches, for example. When a DLCI is provisioned, the network assigns a
Committed Information Rate (CIR), Committed burst (Bc) and Excess burst
(Be) values for the virtual circuit.
Both CIR and Bc values are guaranteed under normal conditions. Excess burst
bandwidth, though, is not guaranteed at all times. You can set the CIR rate on
the XSR with the
frame-relay cir command.
Frame Relay network design assumes that not all users will need all of their
provisioned bandwidth all the time, and that any unused excess capacity can
be borrowed by other customers to send bursts of data exceeding their
Committed burst rate. In this environment, it is possible for multiple users to
contend for the same resources at the same time causing congestion.
If congestion does occur, Frame Relay provides several reactive mechanisms,
including explicit congestion notifications that inform end stations that
congestion exists on the network.
One issue with reactive congestion controls is that congestion has already
occurred. Although congestion is eventually cleared, frames may be lost and
response times reduced. This problem can be solved if network traffic is
limited to avoid congestion in the first place and that is accomplished with
enforced CIR for a PVC.
CIR enforcement also prevents a PVC from hogging all the bandwidth on the
access link - the connection between the access device and the Frame Relay
switch. Without this feature, one VC can use all the access-link bandwidth
before Frame Relay congestion techniques even start up.
Rate Enforcement (CIR) - Traffic Shaping
Traffic shaping is a high level switch to throttle output traffic to address
congestion on the network, enabled by the
frame-relay traffic-shaping
command on the XSR. Adaptive shaping is the ability to further reduce CIR to
alleviate network congestion, enabled by the
frame-relay adaptive-
shaping
command on the XSR.