Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.14.1.0 - Page 748

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 112. Packet Drop Rate for WRED You can create a custom WRED profile or use one of the five pre-defined profiles. Table 78. Pre-Defined WRED Profiles Default Profile Name Minimum Threshold wred_drop 0 wred_teng_y 467 wred_teng_g 467 wred_fortyg_y 467 wred_fortyg_g 467 Maximum Threshold 0 4671 4671 4671 4671 Maximum Drop Rate 100 100 50 50 25 748 Quality of Service (QoS)