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Understanding QoS
This example uses a policy map to remark a packet. The first marking (the set command) applies to the
QoS default class map that matches all traffic not matched by class AF31-AF33 and sets all traffic to an
IP DSCP value of 1. The second marking sets the traffic in classes AF31 to AF33 to an IP DSCP of 3.
Switch(config)# policy-map Example
Switch(config-pmap)# class class-default
Switch(config-pmap-c)# set ip dscp 1
Switch(config-pmap-c)# exit
Switch(config-pmap)# class AF31-AF33
Switch(config-pmap-c)# set ip dscp 3
Switch(config-pmap-c)# exit
Switch(config-pmap)# exit
Switch(config)# interface fastethernet0/1
Switch(config-if)# service-policy input Example
Switch(config-if)# exit
Congestion Management and Scheduling
MQC provides several related mechanisms to control outgoing traffic flow. These mechanisms are
implemented in output policy maps to control output traffic queues. The scheduling stage holds packets
until the appropriate time to send them to one of the four 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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Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
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Traffic 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.
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 a percentage of
total bandwidth or of remaining bandwidth.
Priority queuing or class-based priority queuing
You use the priority policy-map class configuration command to specify the priority of a type of
traffic over other types of traffic. You can specify strict priority for the high-priority traffic and
allocate any excess bandwidth to other traffic queues, or specify priority with unconditional policing
of high-priority traffic and allocate the known remaining bandwidth among the other traffic queues.
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To configure strict priority, use only the priority policy-map class configuration command to
configure the priority queue. Use the bandwidth remaining percent policy-map class
configuration command for the other traffic classes to allocate the excess bandwidth in the
desired ratios.
To configure priority with unconditional policing, configure the priority queue by using the
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priority policy-map class configuration command and the police policy-map class
configuration command to unconditionally rate-limit the priority queue. In this case, you can
configure the other traffic classes with bandwidth or shape average, depending on
requirements.
Chapter 30
Configuring QoS
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Troubleshooting
