Cisco ME 3400 Software Configuration Manual page 654

Understanding QoS
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.
When you configure a queue limit for a class in an output policy map, all other output policy maps must
Note
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.
When you use the queue-limit command to configure queue thresholds for a class, the WTD thresholds
Note
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:
Note
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.
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
33-28
Chapter 33 Configuring QoS
OL-9639-07