Cisco IE-4000 Software Configuration Manual page 603

Configuring QoS
QoS Treatment for Performance-Monitoring Protocols
Switch(config-if)# exit
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. You can attach only three unique
queue-limit configurations in output policy maps to interfaces at any one time. 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.
Note: When you configure a queue limit for a class in an output policy map, all other output policy maps must use the
same qualifier type and qualifier value format. Only the queue-limit threshold values can be different. For example, when
you configure class A queue limit thresholds for dscp 30 and dscp 50 in policy map PM1, and you configure class A
queue limits in policy map PM2, you must use dscp 30 and dscp 50 as qualifiers. You cannot use dscp 20 and dscp 40.
The threshold values can be different, but different threshold values creates a new queue-limit configuration.
By default, the total amount of buffer space is divided equally among all ports and all queues per port, which is adequate
for many applications. You can decrease the queue size for latency-sensitive traffic or increase the queue size for bursty
traffic.
Note: When you use the queue-limit command to configure queue thresholds for a class, the WTD thresholds must be
less than or equal to the queue maximum threshold. A queue size configured with no qualifier must be larger than any
queue sizes configured with qualifiers.
When you configure queue limit, the range for the number of packets is from 16 to 544, in multiples of 16, where each
packet is a fixed unit of 256 bytes.
Note: For optimal performance, we strongly recommend that you configure the queue-limit to 272 or less.
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.

A small buffer reduces latency but is more appropriate for steady traffic flows than for bursty traffic.

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.
These restrictions apply to WTD qualifiers:

You cannot configure more than two threshold values for WTD qualifiers (cos, dscp, precedence, qos-group) by
using the queue-limit command. However, there is no limit to the number of qualifiers that you can map to these
thresholds. You can configure a third threshold value to set the maximum queue by using the queue-limit command
with no qualifiers.

A WTD qualifier in the queue-limit command must be the same as at least one match qualifier in the associated
class map.
This example shows how to configure bandwidth and queue limit so that out-class1, out-class2, out-class3, and
class-default get a minimum of 40, 20, 10 and 10 percent of the traffic bandwidth, respectively. The corresponding
queue-sizes are set to 48, 32, 16 and 272 (256-byte) packets:
Switch(config)# policy-map out-policy
Switch(config-pmap)# class outclass1
Switch(config-pmap-c)# bandwidth percent 40
Switch(config-pmap-c)# queue-limit 48
Switch(config-pmap-c)# exit
Switch(config-pmap)# class outclass2
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