IBM SC30-3681-08 manual Variable Information Rate, Circuit Congestion, CIR Monitoring

Using Frame Relay

If the CIR monitoring feature is not enabled the maximum information rate is equal to the line speed.

Variable Information Rate

The variable information rate (VIR) ranges from the con®gured minimum IR to the calculated maximum IR when the CIR monitoring or congestion monitoring features are enabled. The VIR is gradually decreased down to the minimum information rate when the router is noti®ed of congestion on a circuit and is gradually increased to the maximum information rate when the router stops receiving congestion noti®cations. Using theset ir-adjustmentcon®guration command, you con®gure the percentage of the information rate by which the VIR should decrease when the router is noti®ed of congestion. You also use this command to con®gure the percentage of the information rate by which the VIR should be gradually increased when the congestion ends.

To avoid impulse loading of the network, the router initially sets the VIR to CIR when the VC becomes active. If you con®gure 0 for CIR, VIR is initially set to excess burst (Be) times the MIR adjustment percentage. For example, if Be is set to 64 000 and the MIR adjustment percentage is set to 25%, then the initial VIR would be equal to 16 000 bps.

The VIR can actually exceed the maximum value in one case. If the length of a frame in bits is greater than the maximum IR, Frame Relay transmits the frame anyway.

Circuit Congestion

Circuit congestion occurs for one of the following reasons:

vThe sender is transmitting faster than the allowable throughput

vThe receiver is too slow when processing the frames

vAn intermediate backbone link is congested, resulting in the sender transmitting faster than the available throughput allows.

When circuit congestion happens, the network must drop packets and/or shut down.

In response to circuit congestion, the router implements a throttle down, which is a step-wise slowing of packet transmission to the con®gured minimum IR. Throttle down occurs during the following conditions:

vCircuit congestion is occurring.

vThe router is the sender of frames.

vCIR monitoring or congestion monitoring is enabled.

This section discusses monitoring of Frame Relay data rates and circuit congestion.

CIR Monitoring

CIR monitoring is an optional Frame Relay feature that you can set for each interface to prevent the router from creating congestion conditions in the FR

network. CIR monitoring allows the VIR for a VCto range between the con®gured minimum and maximum IR.

CIR monitoring is con®gured with theenable cir-monitorcon®guration command and is disabled by default. CIR monitoring, when enabled, overrides congestion

Chapter 30. Using Frame Relay Interfaces 399