The next two sections in this chapter cover WRED and FRED, including their respective configurations, as well as comparing RED, WRED, and FRED.

Weighted RED (WRED)

WRED behaves almost identically to RED, as described in the preceding section of this chapter. It calculates the average queue depth, and decides whether to discard packets, and what percentage of packets to discard, based on all the same variables as RED. The only real difference between the two is that WRED weights its behavior based on the IP precedence or IP differentiated services code point (DSCP) values of packets.

The other major concept that needs to be covered, before diving into WRED configuration, relates to where WRED can be enabled, and how it interoperates with queuing tools. Interestingly, although WRED can be enabled on an interface, it cannot be concurrently enabled along with any other queuing tool! When using Modular QoS command-line interface (MQC) to configure queuing, however, WRED can be used for individual class queues.

The following sections cover the following:

How WRED Weights Packets

WRED bases its decisions about when to discard packets, and what percentage to discard, on the following four factors:

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First, just like RED, WRED calculates the average queue depth. WRED then compares the average queue depth to the minimum and maximum thresholds to decide whether it should discard packets. If the average queue depth is between the two thresholds, WRED discards a percentage of the packets, with the percentage based on the MPD; if the average queue depth exceeds the maximum threshold, WRED discards all new packets.

To weight based on precedence or DSCP markings, WRED sets the minimum threshold, maximum threshold, and the MPD to different values per precedence or DSCP value. The average queue depth calculation, however, is not based on the precedence or DSCP value, but is instead calculated for all packets in the queue, regardless of the precedence or DSCP value.

An example of how WRED weights packets can help you make more sense out of how WRED behaves differently than RED. First, consider Figure 6-7, which happens to show the default settings for precedence 0.

Figure 6-7 Default WRED Settings for Precedence 0 for Thresholds and Percent to Discard

Discard

Percentage

100%

WRED calculates the average queue depth just like RED, ignoring precedence, but it decides when to discard packets based on the precedence or DSCP value. Suppose, for instance, that the average queue depth just passed 20. For new precedence 0 packets that need to be placed into the queue, WRED begins discarding some packets. If the average queue depth continues to increase toward 40, WRED continues to discard precedence 0 packets, but more aggressively, up to a rate of 10 percent, when the average queue depth reaches 40. After the average queue depth passes 40, WRED discards all new precedence 0 packets. In fact, if all packets were precedence 0, RED and WRED would behave identically.

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IOS uses logical choices for the default settings for all WRED parameters. However, you can choose to override the parameters with configuration commands. Tables 6-4 and 6-5 list the IOS default values for minimum threshold, maximum threshold, and MPD with precedence-based WRED (Table 6-4) and DSCP-based WRED (Table 6-5).

Table 6-4 Cisco IOS Software-Default Values for Precedence-Based WRED

*RSVP = Resource Reservation Protocol

Table 6-5 Cisco IOS Software Default Values for DSCP-Based WRED*

*Stated values for IOS 12.2 Mainline software.

**Class selector DSCP values use the same values as their corresponding IP precedence values’ settings.

Cisco IOS Software follows the suggested meaning of all DSCP values, including the fact that these four AF DSCP values should be given equal treatment. The last digit of the name of the AF DSCP value identifies the drop preference, with 3 being most likely to be dropped, and 1