Weighted Random Early Detection - Dell Z9500 Configuration Manual

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 Z9500, WRED and Explicit Congestion Notification (ECN) marking are supported on front-end I/O and
backplane high-Gigabit 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 to finely tune how different types of traffic are handled when a WRED threshold is exceeded, see
Configuring Weights and ECN for WRED
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.
Quality of Service (QoS) 680