Content

| 8.5 |  | Scheduling |
| 8.5.6 |  | Configuring CBWFQ |

In this example incoming traffic that has been marked with a DSCP of 50 will be given 40 percent of the outbound links available bandwidth. All other traffic will share the remaining 60 percent of bandwidth using WFQ. Configuring CBWFQ begins with turning on QoS functionality and defining the traffic that belongs to a particular class.

> Router(config)#mls qos
> Router(config)#class-map prioritytraffic
> Router(config)#match dscp 50

Next a policy-map defining the QoS requirements for both the priority traffic identified and all other traffic, default, is created.

> Router(config)#policy-map prioritybw
> Router(config-pmap)#class class-default fair-queue
> Router(config-pmap-c)#class prioritytraffic bandwidth percent 40 queue-limit 200

The queue-limit specifies the depth of the queue for that class. Specifying a queue-limit indicates to the router that a tail-drop policy is to be used. Inbound traffic is placed in the queue until the queue is full. Once the queue is full all further packets are dropped until space is again available in the queue. In the next section an alternative handling of congested queues is explored.

Finally the policy has to be actually implemented by applying it to an interface.

> Router(config)#interface gigabitethernet0/1
> Router(config-if)#service-policy output prioritybw

Configuring CBWFQ on a physical interface is only possible if the interface is in the default queuing mode. Serial interfaces at E1 (2.048 Mbps) and below use WFQ by default—other interfaces use FIFO by default. Enabling CBWFQ on a physical interface overrides the default interface queuing method.

> Note: Class Based Weighted Fair Queuing can not be configured on the Catalyst 2950 or 3550 Switches. However it is supported on Catalyst 6500 switches and IOS based routers.

Show Commands and Purpose
Router#show policy-map policy-map – Displays the configuration of all classes that make up the specified policy map.

Router#show policy-map policy-map class class-name – Displays the configuration of the specified class of the specified policy map.

Router#show policy-map interface interface-name – Displays the configuration of all classes configured for all policy maps on the specified interface.

Router#show queue interface-type interface-number – Displays queuing configuration and statistics for a particular interface.

The counters displayed after issuing the show policy-map interface command are updated only if congestion is present on the interface.