WRED versus Tail Drop (Egress Congestion Avoidance)

Posted: May 9, 2006 by sankar in QOS

WRED uses random dropping of packets. When threshold 1 is reached (say 25% of queue) packets with Cos 0 and 1 are randomly dropped. When threshold 2 is reached (say 50% of queue), packets with CoS 0,1,2,3 are randomly dropped. At this point packets with Cos 0,1 are more aggresively dropped than packets with Cos 2,3.

With Tail drop, assume a queue has 4 thresholds. (10% 25% 50% and 80%). Referring to the picture, if the queue is 10% full, packets with Cos 0 and 1 are dropped. When it is 25% full, packets with Cos 0,1,2,3 are dropped. and so on.

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