Priority Queuing - Cisco ME 3400 Software Configuration Manual
Ethernet access switch
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- Command reference manual (950 pages) ,
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Chapter 33
Configuring QoS
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.
You should exercise care when using the priority command. Excessive use of strict priority queuing
Note
might cause congestion in other queues.
The switch supports strict priority queuing or priority used with the police policy-map command.
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Priority queuing has these restrictions:
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For more information, see the
section on page
This example shows how to configure the class out-class1 as 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-2
gets 50 percent of the remaining bandwidth and out-class3 gets 20 percent of the remaining bandwidth.
The class class-default receives the remaining 30 percent with no guarantees.
Switch(config)# policy-map policy1
Switch(config-pmap)# class out-class1
Switch(config-pmap-c)# priority
Switch(config-pmap-c)# exit
Switch(config-pmap)# class out-class2
Switch(config-pmap-c)# bandwidth remaining percent 50
Switch(config-pmap-c)# exit
OL-9639-07
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.
You cannot configure priority without policing for a traffic class when traffic shaping or
Note
CBWFQ are configured for another class in the same output policy map.
You can use priority with the police 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.
Note
When priority is configured in an output policy map without the police command, you can
only configure the other queues for sharing by using the bandwidth remaining percent
policy-map command to allocate excess bandwidth.
You can associate the priority command with a single unique class for all attached output polices
on the switch.
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-default of an output policy map.
33-58.
"Configuring Output Policy Maps with Class-Based Priority Queuing"
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
Understanding QoS
33-25
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