Deployment Guide

Weighted Random Early Detection
Weighted random early detection (WRED) is a congestion avoidance mechanism that drops packets to prevent buffering
resources from being consumed.
NOTE: On the switch, WRED and Explicit Congestion Notification (ECN) marking are supported on front-end I/O and
backplane HiGig ports. When you enable WRED, packets are dropped during times of network congestion based on the
configured minimum and maximum WRED thresholds. ECN marks packets for later transmission (instead of dropping them)
when the network recovers from a heavy traffic condition. For information about how to configure weights for WRED and
ECN operation, see Configuring Weights and ECN for WRED.
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 125. Packet Drop Rate for WRED
You can create a custom WRED profile or use one of the five pre-defined profiles.
Table 85. Pre-Defined WRED Profiles
Wred-profile-name min-threshold max-threshold max-drop-rate
wred_drop 0 0 100
wred_teng_y 594 5941 100
wred_teng_g 594 5941 50
wred_fortyg_y 594 5941 50
wred_fortyg_g 594 5941 25
wred_oneg_y_pe 154 1538 100
wred_oneg_g_pe 154 1538 50
wred_teng_y_pe 154 1538 50
796 Quality of Service (QoS)