Weighted RED (WRED) 455

Example 6-3 WRED Used in LLQ Classes dscp-af21, dscp-af23, and class-default (Continued)

The example lists a large configuration, but only a small amount pertains to WRED. Two sets of class maps have been configured—one set is used by the CB marking policy called laundrylist, and the other set is used by the LLQ policy map called queue-on-dscp. In policy-map queue-on-dscp, inside classes dscp-af21, dscp-af23, and class-default, the random-detect dscp-based command enables WRED. These three random-detect commands are highlighted in the show running-config output in the example.

Also note that WRED is not enabled on interface serial 0/0 in this configuration, because WRED applies to the output queues used by each class. Because WRED is not enabled on the main interface, to see statistics for WRED, you must use the show policy-map interface command. This command in the example lists WRED statistics inside each class in which WRED has been enabled. For the classes in which WRED is not enabled, such as the dscp-ef class, no additional WRED statistical information is listed. The default values for exponential weighting constant, and the per-DSCP defaults for minimum threshold, maximum threshold, and MPD are all listed in the command output.

WRED Summary

WRED provides a valuable tool for managing congestion in queues. Cisco IOS uses defaults that conform to the DiffServ Assured Forwarding conventions, which reduce the likelihood that you will need to configure thresholds for WRED. WRED can be particularly effective when used with MQC-based queuing tools, but when enabled directly on an interface, WRED has the unfortunate side effect of disallowing other queuing tools to be used. Table 6-8 lists some of WRED’s key points.