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Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
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Chapter 33 Configuring QoS Understanding QoS
Class-Based Weighted Fair Queuing
You can configure class-based weighted fair queuing (CBWFQ) to set the relative precedence of a queue
by allocating a portion of the total bandwidth that is available for the port. You use the bandwidth
policy-map class configuration command to set the output bandwidth for a class of traffic as a rate
(kilobits per second), a percentage of total bandwidth, or a percentage of remaining bandwidth.
Note When you configure bandwidth in a policy map, you must configure all rates in the same format, either
a configured rate or a percentage. The total of the minimum bandwidth guarantees (CIR) for each queue
of the policy cannot exceed the total speed of the parent.
• When you use the bandwidth policy-map class configuration command to configure a class of
traffic as an absolute rate (kilobits per second) or a percentage of total bandwidth, this represents
the minimum bandwidth guarantee (CIR) for that traffic class. This means that the traffic class gets
at least the bandwidth indicated by the command, but is not limited to that bandwidth. Any excess
bandwidth on the port is allocated to each class in the same ratio in which the CIR rates are
configured.
Note You cannot configure bandwidth as an absolute rate or a percentage of total bandwidth when
strict priority (priority without police) is configured for another class in the output policy.
• When you use the bandwidth policy-map class configuration command to configure a class of
traffic as a percentage of remaining bandwidth, this represents the portion of the excess bandwidth
of the port that is allocated to the class. This means that the class is allocated bandwidth only if there
is excess bandwidth on the port, and if there is no minimum bandwidth guarantee for this traffic
class.
Note You can configure bandwidth as percentage of remaining bandwidth only when strict
priority (priority without police) is configured for another class in the output policy map.
For more information, see the “Configuring Output Policy Maps with Class-Based-Weighted-Queuing”
section on page 33-53.
Note You cannot configure bandwidth and traffic shaping (shape average) or priority queuing (priority) for
the same class in an output policy map.
This example shows how to set the precedence of output queues by setting bandwidth in kilobits per
second. The classes outclass1, outclass2, and outclass3 and class-default get a minimum of 40000,
20000, 10000, and 10000 kbps. Any excess bandwidth is divided among the classes in the same
proportion as the CIR rated.
Switch(config)# policy-map out-policy
Switch(config-pmap)# class outclass1
Switch(config-pmap-c)# bandwidth 40000
Switch(config-pmap-c)# exit
Switch(config-pmap)# class outclass2
Switch(config-pmap-c)# bandwidth 20000
Switch(config-pmap-c)# exit
Switch(config-pmap)# class outclass3
Switch(config-pmap-c)# bandwidth 10000
Switch(config-pmap-c)# exit