User Manual

Weighted Random Early Detection (WRED)
436 Enterasys X-Pedition User Reference Manual
The ability to dampen the response time for changes to the average queue size changes the way that
WRED responds to bursty traffic. For example, notice in Figure 31 that while the traffic (solid line)
bursts at times, the average queue size (dotted curve) is dampened such that it does not rise above
the minimum threshold within the duration of the bursts. This prevents the port from discriminating
against periodic bursts of traffic.
Figure 31. Average queue size and bursty traffic
The mark-prob-denominator parameter is used to determine the probability of a packet being
dropped when the average queue size is between the minimum and maximum thresholds. With
WRED applied to input queues, the mark-prob-denominator accepts a value of zero (0) to three
(3). If applied to output queues, WRED allows you to assign a value of zero (0) to seven (7). Please
note that the lower the value, the higher the probability of dropping packets.
Both the exponential-weighting-constant value and the mark-prob-denominator value are
somewhat allegorical in the sense that neither value has a direct numerical significance other than
acting as control values for WRED. For example, if you increase the value for exponential-
weighting-constant from 1 to 2, the dampening of the average queue size response is not twice as
slow. Because of the non-specific nature of exponential-weighting-constant and mark-prob-
denominator and the fact that each network is different, a discussion of recommended, specific
values for these parameters is beyond the scope of this text.
When first implementing WRED on an XP, Enterasys Networks recommends that you initially use
the default values for min-threshold, max-threshold, the weighting constant, and the probability
denominator. If you begin to experience congestion or if congestion continues (especially with TCP
traffic), try adjusting WRED by making small changes to the various parameters (one at a time) and
observing the effect on congestion.