Queuing Tools 271

You can understand the usage of the ToS byte by WFQ with a little closer examination of the output of the show queue command. You may recall that the two HTTP transfers were marked with different DSCP values; note that the two HTTP flows in the command output have ToS byte values of 72 and 80. Which of these values corresponds to DSCP AF21 (important HTTP URLs per CB marking) and AF23 (“not” important URLs per CB marking)? Table 4-8 lists the pertinent details needed to correlate DSCP, ToS, and precedence values used in the example.

Table 4-8 DSCP, ToS Byte, and WFQ Weight Values Used in Example 4-3

WFQ always weights the packets based on the first 3 bits of the ToS byte—in other words, based on the Precedence field. However, the show queue command output lists the entire contents of the ToS byte, which in this case included 6 bits marked by CB marking, and two trailing binary 0s. Therefore, the ToS byte values in the command lists the same values shown in the table. Even though CB marking marked a different DSCP for each type of HTTP traffic, as far as WFQ is concerned, each receives the same amount of weight. This is because WFQ does not look past the Precedence field when determining the weight.

Finally, the second half of Example 4-3 just shows some configuration changes and the resulting changes in the show command output. The configuration changes the CDT, the maximum number of queues, and the length of the hold queue. The highlighted portions of the show interface, show queueing, and show queue commands reflect the details of the configuration changes.

WFQ Summary

WFQ works well for networks where the most delay-sensitive traffic requires less bandwidth than the average flow. Also flows with higher precedence work well, with low-volume, highprecedence flows receiving exceptional treatment. Best of all, WFQ requires no classification configuration. As a result, WFQ provides a great default queuing choice, particularly when traffic characteristics are unpredictable and difficult to qualify.