Congestion Avoidance

Congestion avoidance may randomly drop packets from selected queues when previously defined limits are reached.

By dropping packets early, congestion avoidance helps prevent bottlenecks downstream in the network.

Congestion avoidance technologies include random early detection and weighted random early detection.

Congestion-avoidance mechanisms monitor network traffic loads in an effort to anticipate and avoid congestion at common network bottlenecks. Congestion avoidance is achieved through packet dropping.

Congestion avoidance mechanisms are typically implemented on output interfaces wherever a high-speed link or set of links feeds into a lower-speed link (such as, a LAN feeding into a slower WAN link.) This ensures that the WAN is not instantly congested by LAN traffic.

Weighted random early detection (WRED) is a Cisco primary congestion-avoidance technique. WRED increases the probability that congestion is avoided by dropping low-priority packets rather than dropping high-priority packets.

WRED is not recommended for voice queues. A network should not be designed to drop voice packets.

Note The tools for congestion avoidance are covered in detail in the "Congestion Avoidance"

module in this course.

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