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Cisco ME 3800X and 3600X Switch Software Configuration Guide
OL-23400-01
Chapter 27 Configuring QoS
Understanding QoS
Figure 27-7
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 microseconds worth of data, and a new frame arrives containing
CoS values 4 or 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.
You configure WTD by using the queue-limit policy-map class command to adjust the queue size (buffer
size) associated with a particular class of traffic.
Note
Queue-limit is supported only in leaf-level (per-hop behavior) classes.
You specify the maximum threshold in bytes or in microseconds. You can specify different queue sizes
for different classes of traffic (CoS, DSCP, MPLS EXP, precedence, discard-class, 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 (including the default) across all
output policy maps. Within an output policy map, four or eight queues (classes) are allowed, including
the class default. Each queue can have three defined thresholds. Only three unique threshold value
configurations are allowed per class. Multiple policy maps can share the same queue-limits. Each
class-map in a policy-map can either share the same threshold or can have its own unique values.
You can use these same queue-limit values in multiple output policy maps on the switch. However,
changing one of the queue-limit values in a class creates a new, unique queue-limit configuration. At any
one time, you can attach only three unique queue-limit configurations in output policy maps to targets.
If you attempt to attach an output policy map with a fourth unique queue-limit configuration, you see
this error message:
QoS: Configuration failed. Maximum number of allowable unique queue-limit
configurations exceeded.
By default, queues have unique thresholds based on the speed of the interface. You can decrease the
queue size for latency-sensitive traffic or increase the queue size for bursty traffic.
Queue bandwidth and queue size (queue limit) are configured separately and are not interdependent. You
should consider the type of traffic being sent when you configure bandwidth and queue-limit:
•
A large buffer (queue limit) can better accommodate bursty traffic without packet loss, but at the
cost of increased latency.
CoS 6-7
100%
60%
40%
1000
600
400
0
CoS 4-5
CoS 0-3
86692
