3Com Switch 8800 Advanced Software V5 Configuration Guide

Configuring WRED 861
Through associating WRED with WFQ, the flow-based WRED can be realized.
Because different flow has its own queue during packet classification, the flow
with small traffic always has a small queue length, so the packet drop probability is
low. The flow with big traffic has bigger queue length, so more packets are
dropped. In this way, the benefits of the flow with small traffic are protected.
Configuring WRED
Description on WRED
Parameters
Determine the following parameters before configuring WRED:
■ The maximum threshold and minimum threshold: when the average queue
length is smaller than the minimum threshold, no packet is dropped. When the
average queue length is between the maximum threshold and the minimum
threshold, the packets are dropped randomly. The longer the queue is, the
higher the drop probability is. When the average queue length exceeds the
average queue length, all the incoming packets are dropped.
■ The exponent used for calculating average queue length: the bigger the
exponent is, the more sensitive to real-time queue change the average queue
length is.
■ The denominator used for calculating the drop probability: this argument
functions as the denominator when the drop probability is calculated. The
bigger the denominator is, the smaller the calculated drop probability is.
n
■ 3C17532 and 3C17538 modules do not support WRED.
■ WRED cannot be configured on ports with even port numbers on 3C17526
modules, and WRED configured on a port with an odd port number takes
effect on the incoming packets of both the port with the odd port number and
the port with the even port number (the odd port number plus one).
Configuration Procedure Configuring and applying a queue-based WRED table
Follow these steps to configure and apply a queue-based WRED table:
To do... Use the command... Remarks
Enter system view system-view -
Configure a WRED table qos wred queue table
table-name
Required
Configure the exponent used for
calculating average queue length
queue queue-value
weighting-constant
exponent
Optional
9 by default
Configure the other parameters for the
WRED table
queue queue-value [
drop-level drop-level ]
low-limit low-limit
high-limit high-limit [
discard-probability
discard-prob ]
Optional
By default, the low-limit
argument is 10, the
high-limit argument is
30, and the discard-prob
argument is 10.