Cisco Cookbook

Chapter 11. Queueing and Congestion

11.0. Introduction

Quality of Service (QoS) has been a part of the IP protocol since RFC 791 was released in 1981. However, it has not been extensively used until recently. The main reason for using QoS in an IP network is to protect sensitive traffic in congested links. In many cases, the best solution to the problem of congested links is simply to upgrade them. All you can do with a QoS system is affect which packets are forwarded and which ones are delayed or dropped when congestion is encountered. This is effective only when the congestion is intermittent. If a link is just consistently over- utilized, then QoS will at best offer a temporary stopgap measure until the link is upgraded or the network is redesigned.

There are several different traffic flow characteristics that you can try to control with a QoS system. Some applications require a certain minimum throughput to operate, while others require a minimum latency. Jitter, which is the difference in latency between consecutive packets, has to be carefully constrained for many real-time applications such as voice and video. Some applications do not tolerate dropped packets well. Others contain time-sensitive information that is better dropped than delayed.

There are essentially three steps to any traffic prioritization scheme. First, you have to know what your traffic patterns look like. This means you need to understand what traffic is mission critical, what can wait, and what traffic flows ...