488 BigIron RX Series Configuration Guide
53-1002253-01
Determining packet drop priority using WRED
18
How WRED Operates
The graph in Figure 88 describes the interaction of the previously described variables in the
operation of WRED. When a packet arrives at a switch, the average queue size (q-size) is calculated
(note that this is not the statistical average queue size - (refer to “Calculating avg-q-size” on
page 488). If q-size as calculated is below the configured Min. Average Queue Size, then the packet
is accepted. If the average queue size is above the configured Max. Average Queue Size threshold,
the packet is dropped. If the Average Queue size falls between the Min. Average Queue Size and
the Max. Average Queue Size, packets are dropped according to the calculated probability
described in “Calculating packets that are dropped” on page 488.
FIGURE 88 WRED operation graph
Calculating avg-q-size
The algorithm first calculates the avg-q-size through the following equation.
avg-q-size = ( (1 - Wq) * Statistical Average-Q-Size) + (Wq * Current-Q-Size)
The Wq value is instrumental to the calculation and can be:
• equal to the statistical average queue size (Wq == 0), or
• equal to the current queue size (Wq == 1) or
• be between 0 and 1 (0 < Wq < 1).
Lower Wq values cause the avg-q-size to lean towards the statistical average queue size, reducing
WRED's sensitivity to the current state of the queue and thus reduce WRED's effectiveness. On the
other hand, higher Wq values cause the avg-q-size to lean towards the instantaneous queue size,
which exposes WRED to any change in the instantaneous queue size and thus may cause WRED to
overreact in cases of bursts. Thus, the value of Wq should be carefully chosen according to the
application at hand.
Calculating packets that are dropped
The Pdrop value, as calculated in the following equation, is the probability that a packet will be
dropped in a congested switch.
Pmax
Min. Average
Queue Size
Max. Average
Queue Size
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