As previously discussed, IP Precedence marking has been made obsolete by Differentiated Services, and as such, tools based on IPP, such as WRED, need to be modified accordingly. To achieve DiffServ compliance, the WRED algorithm can be optionally modified to operate based on the Assured Forwarding (AF) drop-preference values (as defined in RFC 2597) to influence its drop probability as queues fill. Such operation is referred to as DSCP-based WRED. Remember, the second digit of an AF codepoint indicates drop preference and can range from 1 (lowest drop preference) to 3 (highest drop preference). For example, if DSCP-based WRED is enabled on a queue servicing AF class 2 traffic, AF23 would be statistically dropped more often than AF22, which, in turn, would be statistically dropped more often than AF21. Figure 1 illustrates a simplified example of DSCP-based WRED for AF class 2.
DSCP-based WRED can be enabled on a CBWFQ—with or without a FQ pre-sorter—with the dscp-based keyword in conjunction with the random-detect command, as demonstrated in Example 1.
Example 1: Enabling DSCP-Based WRED on a CBWFQ
Router(config)# policy-map DSCP-WRED Router(config-pmap)# class TCP Router(config-pmap-c)# bandwidth percent 30 Router(config-pmap-c)# random-detect dscp-based Router(config-pmap-c)# class UDP Router(config-pmap-c)# bandwidth percent 20 Router(config-pmap-c)# class class-default Router(config-pmap-c)# fair-queue Router(config-pmap-c)# random-detect dscp-based
Additionally, DSCP-based WRED thresholds and mark probability denominators are tunable on a per-codepoint basis using the dscp keyword in conjunction with the random-detect command. In Example 2, the minimum threshold is set to begin dropping AF11-marked packets at 5 (meaning as soon as the queue depth reaches 5 packets, WRED randomly begins dropping AF11 packets). The maximum threshold for AF11 (at which all AF11-marked packets drop) is set to 20 packets. And the mark probability denominator is set to 8 (meaning that between these thresholds, up to 1 in 8 packets that are marked with AF11 drop).
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