Priority Queuing - Cisco IE-4000 Software Configuration Manual

Configuring QoS
QoS Treatment for Performance-Monitoring Protocols
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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.
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.
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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.
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
Note: You cannot configure bandwidth and traffic shaping (shape average) or priority queuing (priority) for the same
class in an output policy map.
Note: When you configure CIR bandwidth for a class as an absolute rate or percentage of the total bandwidth, any excess
bandwidth remaining after servicing the CIR of all the classes in the policy map is divided among the classes in the same
proportion as the CIR rates. If the CIR rate of a class is configured as 0, that class is also not eligible for any excess
bandwidth and as a result receives no bandwidth.

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.
Note: You should exercise care when using the priority command. Excessive use of strict priority queuing 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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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 CBWFQ are configured
for another class in the same output policy map.
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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.
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.
Priority queuing has these restrictions:
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You can associate the priority command with a single unique class for all attached output polices on the switch.
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You cannot configure priority and any other scheduling action (shape average or bandwidth) in the same class.
Configuring Output Policy Maps with Class-Based-Weighted-Queuing, page
596
630.