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Cisco ME 3800X and 3600X Switch Software Configuration Guide
OL-23400-01
Chapter 27 Configuring QoS
Understanding QoS
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A small buffer reduces latency but is more appropriate for steady traffic flows than for bursty traffic.
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Very small buffers are typically used to optimize priority queuing. For traffic that is priority queued,
the buffer size usually needs to accommodate only a few packets; large buffer sizes that increase
latency are not usually necessary. For high-priority latency-sensitive packets, configure a relatively
large bandwidth and relatively small queue size.
WTD thresholds:
•
You cannot use the queue-limit command to configure more than two threshold values for WTD
qualifiers (cos, dscp, precedence, qos-group, discard-class, or mpls experimental). However,
there is no limit to the number of qualifiers that you can map to these thresholds.
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You can configure a third threshold value to set the maximum queue by using the queue-limit
command with no qualifiers.
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You can configure many more WTD thresholds, provided their threshold values are equal to the
maximum threshold of the queue provided by the queue-limit command.
See the
“Configuring Weighted Tail Drop” section on page 27-50
.
Congestion Management and Scheduling
MQC provides several related mechanisms in output policy maps to control outgoing traffic flow. The
scheduling stage holds packets until the appropriate time to send them to one of the four or eight traffic
queues. Queuing assigns a packet to a particular queue based on the packet class and is enhanced by the
WTD algorithm for congestion avoidance. You can use different scheduling mechanisms to provide a
guaranteed bandwidth to a particular class of traffic while also serving other traffic in a fair way. You
can limit the maximum bandwidth that can be consumed by a particular class of traffic and ensure that
delay-sensitive traffic in a low-latency queue is sent before traffic in other queues.
The switch supports these scheduling mechanisms:
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Traffic shaping
You use the shape average policy map class configuration command to specify that a class of traffic
should have a maximum permitted average rate. You specify the maximum rate in bits per second.
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Class-based-weighted-fair-queuing (CBWFQ)
You can use the bandwidth policy-map class configuration command to control the bandwidth
allocated to a specific class. Minimum bandwidth can be specified as a bit rate, or as a percentage
of total bandwidth, or as remaining bandwidth.
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Priority queuing or class-based low-latency scheduling
You use the priority policy-map class configuration command to specify the that a class of traffic
has low latency requirements with respect to other classes. When you configure this command in a
class, you cannot configure bandwidth in any other child class associated with the same parent.
These sections contain additional information about scheduling:
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Traffic Shaping, page 27-18
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Class-Based Weighted Fair Queuing, page 27-19
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Priority Queuing, page 27-20
