Users Guide

Weighted Random Early Detection
Weighted random early detection (WRED) is a congestion avoidance mechanism that drops packets to prevent buering resources
from being consumed.
The WRED congestion avoidance mechanism drops packets to prevent buering resources from being consumed.
Trac is a mixture of various kinds of packets. The rate at which some types of packets arrive might be greater than others. In this
case, the space on the buer and trac manager (BTM) (ingress or egress) can be consumed by only one or a few types of trac,
leaving no space for other types. You can apply a WRED prole to a policy-map so that specied trac can be prevented from
consuming too much of the BTM resources.
WRED uses a prole to specify minimum and maximum threshold values. The minimum threshold is the allotted buer space for
specied trac, for example, 1000KB on egress. If the 1000KB is consumed, packets are dropped randomly at an exponential rate
until the maximum threshold is reached (as shown in the following illustration); this procedure is the “early detection” part of WRED.
If the maximum threshold, for example, 2000KB, is reached, all incoming packets are dropped until the buer space consumes less
than 2000KB of the specied trac.
Figure 115. Packet Drop Rate for WRED
You can create a custom WRED prole or use one of the ve pre-dened proles.
Creating WRED Proles
To create WRED proles, use the following commands.
1. Create a WRED prole.
CONFIGURATION mode
wred-profile
2. Specify the minimum and maximum threshold values.
WRED mode
threshold
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Quality of Service (QoS)