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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 configured 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 notified of congestion on a circuit and is gradually increased to
the maximum information rate when the router stops receiving congestion
notifications. Using the set ir-adjustment configuration command, you configure the
percentage of the information rate by which the VIR should decrease when the
router is notified of congestion. You also use this command to configure 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 configure 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:
v The sender is transmitting faster than the allowable throughput
v The receiver is too slow when processing the frames
v An 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 configured minimum IR. Throttle
down occurs during the following conditions:
v Circuit congestion is occurring.
v The router is the sender of frames.
v CIR 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 configured
minimum and maximum IR.
CIR monitoring is configured with the enable cir-monitor configuration command
and is disabled by default. CIR monitoring, when enabled, overrides congestion
Using Frame Relay
Chapter 30. Using Frame Relay Interfaces 399
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