Configuring CBWRED

This topic describes the Cisco IOS commands that are required to configure CB-WRED.

To configure CB-WRED (WRED at the class level with CBWFQ), configure the DSCP-based and precedent-based arguments within MQC. Specific CB-WRED configuration arguments are applied within a policy map. You can then apply the policy map configuration wherever policy maps are attached (for example, at the interface level, the per-virtual circuit [VC] level, or the shaper level).

Configuring CB-WRED (Cont.)



• Enables IP precedence-based WRED in the selected class within the service policy configuration mode.

• Default service profile is used.

• Command can be used at the interface, per-VC (with random-detect-group), or the class level (service-policy).

• Precedence-based WRED is the default mode.

• WRED treats non-IP traffic as precedence 0.

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Use the random-detect command to enable WRED on an interface. By default, WRED is precedence-based and uses eight default WRED profiles, one for each value of IP precedence.

Within the CBWFQ system, WRED is used to perform per-queue dropping within the class queues. Therefore, each class queue has its own WRED method, which can be further weighed based on the IP precedence or DSCP value. Each queue can therefore be configured with a separate dropping policy to implement different drop policies for every class of traffic.

WRED will treat all non-IP traffic as precedence 0. As a result, non-IP traffic is more likely to be dropped than IP traffic.

If the random-detect command is used on versatile interface processor (VIP)-based interfaces, distributed WRED (DWRED) is enabled and the VIP CPU is responsible for WRED dropping. This can significantly increase router performance when used in the context of distributed Cisco Express Forwarding (CEF) switching, which is a prerequisite for DWRED functionality. Also, DWRED can be combined with distributed weighted fair queuing (DWFQ), enabling truly distributed queuing and congestion avoidance techniques, running independently from the central CPU.

WRED cannot be configured on the same interface as custom queuing (CQ), priority queuing (PQ), or WFQ. However, both DWRED and DWFQ can be configured on the same interface. In addition, CB-WRED can be configured in conjunction with CBWFQ. Restricting nondistributed, non-class-based WRED to only FIFO queuing on an interface is typically not a major issue because WRED is usually applied in the network core, where advanced queuing mechanisms are not typically used. WRED is suited for the network core because WRED has a relatively low performance impact on routers. Furthermore, DWRED or CB-WRED can be used to overcome this limitation by combining WRED with WFQ.

When WRED is enabled, default values are selected for each traffic profile based on the weight used (IP precedence or DSCP). You can then modify these default values to match their specific administrative QoS policy goals. When modifying the default WRED profile for IP precedence, these values are configurable:

■ Minimum threshold: When the average queue depth is above the minimum threshold, WRED starts dropping packets. The rate of packet drop increases linearly as the average queue size increases, until the average queue size reaches the maximum threshold. The default maximum threshold is equal to the default hold queue size (40) on an interface. The size of the hold queue is equivalent to the number of packets that can be held within a queue. The hold queue length ranges from 0 to 4096, and, therefore, the minimum/maximum threshold range is 1 to 4096. The default maximum threshold will reflect the defined hold queue size. Thus, if the hold queue is changed, the maximum threshold will change.

■ Maximum threshold: When the average queue size is above the maximum threshold, all packets are dropped. If the difference between the maximum threshold and the minimum threshold is too small, many packets might be dropped at once, resulting in global synchronization.

■ Mark probability denominator: This is the fraction of packets dropped when the average queue depth is at the maximum threshold. For example, if the denominator is 10, one out of every 10 packets is dropped when the average queue is at the maximum threshold. The maximum probability of drop at the maximum threshold can be expressed as 1 / mark-prob-denominator. The maximum drop probability is 10 percent if default settings are used that have a mark probability denominator value of 10. The value of the mark probability can range from 1 to 65536.

If required, you can configure RED as a special case of WRED, by assigning the same profile to all eight IP precedence values. The default WRED parameter is based on the best available data. Cisco recommends that these parameters not be changed from their default values unless you have determined that your applications will benefit from the changed values.

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