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Catalyst 3750 Metro Switch Command Reference
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Chapter 2 Catalyst 3750 Metro Switch Cisco IOS Commands
random-detect exponential-weighting-constant
If you select a high value for the exponential weighting constant, it smooths the peaks and valleys in the
queue size. The average queue size is unlikely to change very quickly, avoiding drastic swings in size.
The WRED process will be slow to start dropping packets, but it might continue dropping packets for a
time after the actual queue size has fallen below the minimum threshold. The slow-moving average
accommodates temporary bursts in traffic.
If you select a low value for the exponential weighting constant, the average queue size closely tracks
the current queue size. The resulting average might fluctuate with changes in the traffic levels. In this
case, the WRED process responds quickly to long queues. When the queue falls below the minimum
threshold, the process stops dropping packets. If the value is too low, WRED overreacts to temporary
traffic bursts and drops traffic unnecessarily.
Note The default WRED exponential weighting constant value is based on the best available data. We
recommend that you do not change the parameters from their default values unless you have decided that
your applications would benefit from the changed values.
To return to policy-map configuration mode, use the exit command. To return to privileged EXEC mode,
use the end command.
Examples This example shows how to configure policy for a class called acl10 included in the policy map called
policy10. Class acl101 has these characteristics: a minimum of 2000 kbps of bandwidth are expected to
be delivered to this class in the event of congestion, and a weight factor of 10 is used to calculate the
average queue size. For congestion avoidance, WRED packet drop is used instead of tail drop. IP
precedence is reset for levels 0 to 2.
Switch(config)# policy-map policy10
Switch(config-pmap)# class acl10
Switch(config-pmap-c)# bandwidth 2000
Switch(config-pmap-c)# random-detect
Switch(config-pmap-c)# random-detect exponential-weighting-constant 10
Switch(config-pmap-c)# random-detect precedence 0 32 256 100
Switch(config-pmap-c)# random-detect precedence 1 64 256 100
Switch(config-pmap-c)# random-detect precedence 2 96 256 100
This example shows how to configure the policy map policy1 to contain policy specification for the class
called class1. During times of congestion, WRED packet drop is used instead of tail drop. The weight
factor used for the average queue-size calculation for the queue for class1 is 12.
Switch(config)# class-map class1
Switch(config-cmap)# match access-group 12
Switch(config-cmap)# exit
Switch(config)# policy-map policy1
Switch(config-pmap)# class class1
Switch(config-pmap-c)# bandwidth 1000
Switch(config-pmap-c)# random-detect
Switch(config-pmap-c)# random-detect exponential-weighting-constant 12
You can verify your settings by entering the show policy-map privileged EXEC command.
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