Congestion avoidance is achieved through packet dropping. Cisco IOS QoS includes an implementation of RED that, when configured, controls when the router drops packets. This chapter gives a brief description of the kinds of congestion avoidance features provided by the Cisco IOS QoS features. This is the default congestion avoidance behavior when WRED is not configured. Weighted Random Early Detection .
Queues fill during periods of congestion.
When the output queue is full and tail drop is in effect, packets are dropped until the congestion is eliminated and the queue is no longer full. This section gives a brief introduction to RED concepts and addresses WRED, Cisco's implementation of RED for standard IOS platforms. WRED avoids the globalization problems that occur when tail drop is used as the congestion avoidance mechanism on the router. Global synchronization occurs as waves of congestion crest only to be followed by troughs during which the transmission link is not fully utilized. Global synchronization manifests when multiple TCP hosts reduce their transmission rates in response to packet dropping, then increase their transmission rates once again when the congestion is reduced.