Cisco ME 3400 Series, Software Configuration Manual

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Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
OL-9639-07
Chapter 33 Configuring QoS
Understanding QoS
Figure 33-7 WTD and Queue Operation
In this example, CoS values 6 and 7 have a greater importance than the other CoS values, and they are
assigned to the 100-percent drop threshold (queue-full state). CoS values 4 and 5 are assigned to the
60-percent threshold, and CoS values 0 to 3 are assigned to the 40-percent threshold.
If the queue is already filled with 600 frames, and a new frame arrives containing CoS values 4 and 5,
the frame is subjected to the 60-percent threshold. When this frame is added to the queue, the threshold
would be exceeded, so the switch drops it.
WTD is configured by using the queue-limit policy-map class command. The command adjusts the
queue size (buffer size) associated with a particular class of traffic. You specify the threshold as the
number of packets, where each packet is a fixed unit of 256 bytes. You can specify different queue sizes
for different classes of traffic (CoS, DSCP, precedence, or QoS group) in the same queue. Setting a queue
limit establishes a drop threshold for the associated traffic when congestion occurs.
Note
You cannot configure queue size by using the queue-limit policy map class command without first
configuring a scheduling action ( bandwidth , shape average , or priority ). The only exception to this is
when you configure queue-limit for the class-default of an output policy map.
The switch supports up to three unique queue-limit configurations across all output policy maps. Within
an output policy map, only four queues (classes) are allowed, including the class default. Each queue has
three thresholds defined. Only three unique threshold value configurations are allowed on the switch.
However, multiple policy maps can share the same queue-limits. When two policy maps a share
queue-limit configuration, all threshold values must be the same for all the classes in both policy maps.
For more information, see the “Configuring Output Policy Maps with Class-Based-Weighted-Queuing”
section on page 33-54 .
This example configures class A to match DSCP values and a policy map, PM1 . The DSCP values of 30
and 50 are mapped to unique thresholds (32 and 64, respectively). The DSCP values of 40 and 60 are
mapped to the maximum threshold of 112 packets.
Switch(config)#
class-map match-any classA
Switch(config-cmap)# match ip dscp 30 40 50 60
Switch(config-cmap)# exit
Switch(config)#
policy-map PM1
Switch(config-pmap)# class classA
Switch(config-pmap-c)# bandwidth percent 50
Switch(config-pmap-c)#
queue-limit 112
Switch(config-pmap-c)# queue-limit dscp 30 32
Switch(config-pmap-c)# queue-limit dscp 50 64
Switch(config-pmap-c)#
exit
Switch(config-pmap)# exit
Switch(config)# interface gigabitethernet 0/1
Switch(config-if)#
service-policy output PM1
Switch(config-if)# exit
CoS 6-7
100%
60%
40%
1000
600
400
0
CoS 4-5
CoS 0-3
86692