Users Guide

DellEMC(conf-qos-policy-in)#set ip-dscp 5
DellEMC(conf-qos-policy-in)#
Weighted Random Early Detection
Weighted random early detection (WRED) is a congestion avoidance mechanism that drops packets to prevent buffering
resources from being consumed.
The WRED congestion avoidance mechanism drops packets to prevent buffering resources from being consumed.
Traffic 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 buffer and traffic manager (BTM) (ingress or egress) can be consumed by only one or a few types
of traffic, leaving no space for other types. You can apply a WRED profile to a policy-map so that specified traffic can be
prevented from consuming too much of the BTM resources.
WRED uses a profile to specify minimum and maximum threshold values. The minimum threshold is the allotted buffer space for
specified traffic, 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 buffer space
consumes less than 2000KB of the specified traffic.
Figure 111. Packet Drop Rate for WRED
You can create a custom WRED profile or use one of the five pre-defined profiles.
Enabling and Disabling WRED Globally
By default, WRED is enabled on the system. You can disable or reenable WRED manually using a single command. Follow these
steps to disable or enable WRED in Dell EMC Networking OS.
● Enable WRED
CONFIGURATION mode
wred enable
● Disable WRED
CONFIGURATION mode
no wred enable
NOTE:
If you disable WRED globally, the system accepts any WRED profile you apply to traffic. But the changes do not
take effect until you enable WRED globally.
Quality of Service (QoS) 703