Cisco Systems A9014CFD manual Priority Queuing, 24-23

Chapter 24 Configuring QoS

Understanding QoS

Router(config)# interface gigabitethernet 0/1

Router(config-if)#service-policy output out-policy

Router(config-if)# exit

Priority Queuing

You can use the priority policy-map class configuration command to ensure that a particular class of traffic is given preferential treatment. With strict priority queuing, the priority queue is constantly serviced. All packets in the queue are scheduled and sent until the queue is empty. Priority queuing allows traffic for the associated class to be sent before packets in other queues are sent.

Caution Be careful when using the priority command. Excessive use of strict priority queuing might cause congestion in other queues.

The router supports strict priority queuing or priority percent policy-map command.

•Strict priority queuing (priority without police) assigns a traffic class to a low-latency queue to ensure that packets in this class have the lowest possible latency. When this is configured, the priority queue is continually serviced until it is empty, possibly at the expense of packets in other queues.

Note You cannot configure priority without policing for a traffic class when traffic shaping or CBWFQ are configured for another class in the same output policy map.

•Use the priority percent policy-map command, or unconditional priority policing, to reduce the bandwidth used by the priority queue. This is the only form of policing that is supported in output policy maps. Using this combination of commands configures a maximum rate on the priority queue, and you can use the bandwidth and shape average policy-map commands for other classes to allocate traffic rates on other queues. From Cisco IOS Release 15.3(2)S, Cisco ASR 901 Router allows configuration of multiple classes to serve based on priority.

Note When priority is configured in an output policy map without the priority command, you can only configure the other queues for sharing by using the bandwidth remaining percent policy-map command to allocate excess bandwidth.

Restrictions

•You can associate the priority command with a single unique class for all attached output polices on the router. From Cisco IOS Release 15.3(2)S, Cisco ASR 901 Router allows configuration of multiple classes with “priority percent.”

•You cannot configure priority and any other scheduling action (shape average or bandwidth) in the same class.

•You cannot configure priority queuing for the class-defaultof an output policy map.

This example shows how to configure the class out-class1as a strict priority queue so that all packets in that class are sent before any other class of traffic. Other traffic queues are configured so that out-class-2gets 50 percent of the remaining bandwidth and out-class3gets 20 percent of the remaining bandwidth. The class class-defaultreceives the remaining 30 percent with no guarantees.

Router(config)# policy-map policy1

Router(config-pmap)# class out-class1

Router(config-pmap-c)# priority

	Cisco ASR 901 Series Aggregation Services Router Software Configuration Guide
OL-23826-09		24-23