CBWFQ Architecture 75 Percent Rule

- Class bandwidths

- RSVP maximum reserved bandwidth

Result must be less than or equal to 75% of interface bandwidth (or Frame Relay, DLCI, CIR)

- Leaves headroom for overhead traffic such as Layer 2 keepalives and bandwidth for the class default traffic

The 75% rule is a conservative rule max-reserved-bandwidth command overrides 75% limit, but seldom recommended

Properly provisioning the network bandwidth is a major component of successful network design. You can calculate the required bandwidth by adding the bandwidth requirements for each major application (for example, voice, video, and data). The resulting sum represents the minimum bandwidth requirement for any given link, and it should not exceed 75 percent of the total available bandwidth for the link (or Frame Relay or ATM PVC committed information Rate [CIR]). This 75 percent rule assumes that some bandwidth is required for overhead traffic, such as layer 2 keepalive messages, as well as for the class default traffic.

The total amount of bandwidth allocated for all classes included in a policy map should not exceed 75 percent of the available bandwidth on the interface. The max-reserved bandwidth command overrides the 75 percent limitation, but overriding is recommended only for the most knowledgeable network administrators who have access to precise figures for available, used, and required bandwidth, including control plane traffic bandwidth requirements. If all of the bandwidth is not allocated, the remaining bandwidth is proportionally allocated among the classes based on the configured bandwidth of the classes.

This topic describes the benefits of CBWFQ.

CBWFQ Benefits

+ Benefits

• Minimum bandwidth allocation

• Finer granularity and scalability

• MQC interface easy to use

• Maximizes transport of priority traffic

• Weights guarantee minimum bandwidth

• Unused capacity shared among the other classes

• Queues separately configured for QoS - Drawbacks

• Voice traffic can still suffer unacceptable delay

CBWFQ allows you to define traffic classes based on custom-defined match criteria such as ACLs, input interfaces, protocol, and QoS label. For example, a class might consist of a team working on a certain project, or a class can be created for the important mission-critical applications such as enterprise resource planning (ERP) applications. When the traffic classes have been defined, they can be assigned a bandwidth, queue limit, or drop policy such as WRED. These are additional benefits of CBWFQ:

■ Bandwidth allocation: CBWFQ allows you to specify the exact amount of bandwidth to be allocated for a specific class of traffic. Accounting for available bandwidth on the interface, you can configure multiple classes (the number depends upon the platform).

■ Finer granularity and scalability: CBWFQ allows you total flexibility to define a class, based on ACLs and protocols or input interfaces, thereby providing finer granularity.

■ Supported by MQC: CBWFQ is supported by the easy-to-use MQC.

The CBWFQ feature is supported on all platforms that WFQ is supported on; in other words, the Cisco 7200, 3800, 3800, 1800, 800, and others.

This topic describes the Cisco IOS commands that are used to configure and monitor CBWFQ on a Cisco router.

Cisco Bandwidth

The bandwidth policy-map class configuration command is used to specify or modify the bandwidth allocated for a class belonging to a policy map.

All classes belonging to one policy map should use the same type of fixed bandwidth guarantee, in kbps, percentage of interface bandwidth, or percentage of available bandwidth.

Configuring bandwidth in percentages is most useful when the underlying link bandwidth is unknown or the relative class bandwidth distributions are known.

bandwidth {bandwidth-kbps | remaining percent percentage | percent percentage}

Syntax Description


Descri ption


Amount of bandwidth, in kbps, to be assigned to the class.

remaining percent percentage

Amount of guaranteed bandwidth, based on a relative percent of available bandwidth. The percentage can be a number from 1 to 100.

percent percentage

Amount of guaranteed bandwidth, based on an absolute percent of available bandwidth. The percentage can be a number from 1 to 100. (By default, only 75 percent can be reserved.)

These restrictions apply to the bandwidth command:

■ If the percent keyword is used, the sum of the class bandwidth percentages cannot exceed 100 percent.

■ The amount of bandwidth configured should be large enough to also accommodate Layer 2 overhead.

■ A policy map can have all the class bandwidths specified in kbps or in percentages, but not a mix of both. However, the unit for the priority command in the priority class can be different from the bandwidth unit of the low-priority class.CBWFQ is not supported on sub-interfaces. If a service policy is applied to a sub-interface in which the service policy directly references a policy map that has a bandwidth statement in it, this error message will be displayed:

router(config-subif)#service-policy output cbwfq CBWFQ : NOT supported on subinterfaces router(config-pmap-c)# queue-limit queue-limit

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