6.2.b [vii] Congestion avoidance [WRED]
Congestion avoidance techniques monitor network traffic loads in an effort to anticipate and avoid congestion at common network bottlenecks. Congestion avoidance is achieved through packet dropping. Among the more commonly used congestion avoidance mechanisms is Random Early Detection (RED), which is optimum for high-speed transit networks. Cisco IOS QoS includes an implementation of RED that, when configured, controls when the router drops packets. If you do not configure Weighted Random Early Detection (WRED), the router uses the cruder default packet drop mechanism called tail drop. 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 of TCP hosts, for example, can occur because packets are dropped all at once. Global synchronization manifests when multiple TCP hosts reduce their transmission rates in response to packet dropping (thus reducing performance), then increase their transmission rates once again when the congestion is reduced.
● Tail drop . This is the default congestion avoidance behavior when WRED is not configured and can lead to global synchronization behavior.
● WRED and distributed WRED (DWRED)— both of which are the Cisco implementations of RED— combine the capabilities of the RED algorithm with the IP Precedence feature.
There are a few variations of WRED:
● Flow-based WRED . Flow-based WRED extends WRED to provide greater fairness to all flows on an interface in regard to how packets are dropped.
● DiffServ Compliant WRED . DiffServ Compliant WRED extends WRED to support Differentiated Services (DiffServ ) and Assured Forwarding (AF) Per Hop Behavior (PHB). This feature enables customers to implement AF PHB by coloring packets according to differentiated services code point (DSCP) values and then assigning preferential drop probabilities to those packets.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 5756-5765). . Kindle Edition.