R2511-HP MSR Router Series ACL and QoS Configuration Guide(V5)

Configuring congestion management

Overview

Causes, impacts, and countermeasures of congestion

Congestion occurs on a link or node when traffic size exceeds the processing capability of the link or

node. It is typical of a statistical multiplexing network and can be caused by link failures, insufficient

resources, and various other causes. Figure 13

shows some common congestion scenarios.

Figure 13 Traffic congestion causes

Congestion can bring the following negative results:

• Increased delay and jitter during packet transmission.

• Decreased network throughput and resource use efficiency.

• Network resource (memory, in particular) exhaustion and system breakdown.

Congestion is unavoidable in switched networks or multiuser application environments. To improve the

service performance of your network, take measures to manage and control it.

One major issue that congestion management deals with is defining a resource dispatching policy to

prioritize packets for forwarding when congestion occurs.

Congestion management policies

Queuing is a common congestion management technique. It classifies traffic into queues and picks out

packets from each queue by using a certain algorithm. Various queuing algorithms are available, and

each addresses a particular network traffic problem. Your choice of algorithm significantly affects

bandwidth assignment, delay, and jitter.

Congestion management involves queue creating, traffic classification, packet enqueuing, and queue

scheduling. Queue scheduling treats packets with different priorities differently to transmit high-priority

packets preferentially.

This section briefly describes several common queue-scheduling mechanisms.