Cisco IOS Cookbook, 2nd Edition

Queueing Algorithms

You can implement several different queueing algorithms on Cisco routers. The most common type is Weighted Fair Queueing (WFQ), which is enabled by default on low-speed interfaces. There is also a class-based version of WFQ called Class-based Weighted Fair Queueing (CBWFQ). These algorithms have the advantage of being fast, reliable, and easy to implement. However, in some cases you might want to consider some of the other queueing systems available on Cisco routers.

Priority Queueing lets you specify absolute prioritization in your network so that more important packets always precede less important ones. This can be useful, but it is often dangerous in practice.

The other important queueing algorithm on Cisco routers is Custom Queueing, which allows you direct control over many of the queueing parameters.

Weighted Fair Queueing

A flow is loosely defined as the stream of packets associated with a single session of a single application. The common IP implementations of Fair Queueing (FQ) and WFQ assume that two packets are part of the same flow if they have the same source and destination IP addresses, the same source and destination TCP or UDP port numbers, and the same IP protocol field value. The algorithms combine these five values into a hash number, and sort the queued packets by this hash number.

The router then assigns sequence numbers to the queued packets. In the FQ algorithm, this process of sequencing the packets is optimized so that each flow gets a roughly ...