Cisco IOS Cookbook, 2nd Edition

Using Weighted Fair Queuing

Problem

You want your routers to use the TOS/DSCP fields when forwarding packets.

Solution

The simplest way to make your routers use DSCP or TOS information is to just make sure that Weighted Fair Queuing is enabled:

Router#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#interface Serial0/0
Router(config-if)#fair-queue
Router(config-if)#exit
Router(config)#end
Router#

WFQ is enabled by default on all interfaces of E1 speeds (roughly 2 Mbps) or less. You can enable WFQ on higher speed interfaces as well, but we don’t recommend it.

To configure more specific behavior, you can tell WFQ how to allocate its queues:

Router#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#interface Serial0/0
Router(config-if)#fair-queue 64 512 10
Router(config-if)#exit
Router(config)#end
Router#

Discussion

Before we discuss these examples in any detail, we should mention that WFQ works well even if you don’t use any TOS/DSCP marking. In this case, it simply gives the same default weighting to every flow, which is the same as conventional Fair Queuing (without the weights). As we discuss in Appendix B, Fair Queuing is much more effective than FIFO queuing. However, with TOS/DSCP marking, WFQ really shows its value, giving higher priority flows more of the bandwidth, and preventing high volume flows from starving low volume flows, regardless of their TOS markings.

The first example just enables ...