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Cisco ME 3800X and 3600X Switch Software Configuration Guide
OL-23400-01
Chapter 27 Configuring QoS
Understanding QoS
You can also configure the PIR in a child policy to be an absolute rate calculated as a percentage of the
PIR of the parent level policy. You can configure the child PIR from 0 percent to 100 percent of the
parent policy.
Switch(config)#
policy-map child
Switch(config-pmap)#
class class2
Switch(config-pmap-c)#
shape average percent 50
Switch(config-pmap-c)#
exit
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 minimum guaranteed output bandwidth or CIR 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.
•
Configuring bandwidth for a class of traffic as an absolute rate (kilobits per second) or a percentage
of total bandwidth represents the minimum bandwidth guarantee (the 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. The CIR range is from 1 Kb/s to 10 Gb/s or 1 to 100 percent.
You configure the bandwidth percent command mainly in hierarchical policy maps where a child
CIR guarantee is tied to the parent CIR guarantee.
The sum of all CIR commitments for a set of peer classes cannot exceed the PIR (shape) of the parent
level. A queue without a configured CIR commitment does not receive any committed bandwidth in
the scheduler and can be entirely superceded other classes. If CIR bandwidth is required for any
class, including class-default, you must configure it.
Note
You cannot configure bandwidth as an absolute rate or a percentage of total bandwidth when
priority is configured for another class in the output policy. However, you can configure
CIR, EIR, and PIR independently for a class and use the bandwidth, bandwidth remaining,
and shape average commands, respectively, at the same time within a class.
•
Configuring bandwidth as a percentage of remaining bandwidth determines the portion of the excess
bandwidth of the target that is allocated to the class. This means that the class is allocated bandwidth
only if there is excess bandwidth on the target and if there is no minimum bandwidth guarantee for
this traffic class. By default, the total excess bandwidth is divided equally among the classes. You
can configure bandwidth as remaining percentage to configure an unequal distribution for
prioritizing classes. The total bandwidth that you can allocate between peer classes is 100 percent.
Note
You cannot configure bandwidth as percentage of remaining bandwidth when priority is
configured for another class in the output policy map.
