Congestion Management

Congestion in a network may occur when users send data at a rate greater than that are acceptable by network resources. For example, congestion may occur because the switches in a network have a limited buffer size of memory to store packets for processing.

            Congestion management is the process of controlling congestion by determining the order in which packets are transmitted out, based on priorities assigned to those packets. Congestion management deals with the creation of queues  based on the packets classification, and scheduling of the packets in the queue for transmission.

            During periods with light traffic, that is, when no congestion exists, packets are transmitted out as soon as they arrive. During periods of heavy traffic, packets arrive faster than the interface can transmit them. If you use congestion management features, packets accumulating at the interface are queued until the interface is free to transmit them; they are then scheduled for transmission according to their assigned priority and the queuing mechanism configured for the interface. The router determines the order of packet transmission by controlling which packets are placed in which queue and how queues are serviced with respect to each other. There are four types of queuing protocols. These are:

  (a)  First-In, First-Out Queuing (FIFOQ)

In FIFO queuing, there no concept of priority or classes of traffic. The transmitted of packets occurs in the order the packets arrive.

  (b) Priority Queuing (PQ)

With PQ, packets belonging to one priority class of traffic are transmitted before all lower priority traffic. This ensures timely delivery of more important packets.

  (c)  Custom Queuing (CQ)

With CQ, bandwidth is allocated proportionally to each class of traffic. CQ allows you to specify the number of bytes or packets to be drown from the queue, which is especially useful on slow interfaces.

 (d) Weighted Fair Queuing (WFQ)

WFQ offers dynamic, fair queuing that divides bandwidth across queues of traffic based on the packet’s weights. Because of its fair handling of bandwidth, WFQ ensures satisfactory response time to critical applications, such as interactive, transaction-based applications, that are intolerant of performance degradation.

When no other queuing strategies are configured, interfaces use FIFO by default.