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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 29 Configuring QoS
Understanding QoS
You assign two drop thresholds to each queue, map DSCPs to the thresholds through the
DSCP-to-threshold map, and enable either tail drop or WRED on the interface. The queue size, drop
thresholds, tail-drop or WRED algorithm, and the DSCP-to-threshold map work together to determine
when and which packets are dropped when the thresholds are exceeded. You configure the drop
percentage thresholds by using either the wrr-queue threshold interface configuration command for tail
drop or the wrr-queue random-detect max-threshold interface configuration command for WRED; in
either case, you map DSCP values to the thresholds (DSCP-to-threshold map) by using the wrr-queue
dscp-map interface configuration command. For more information, see the
and
.
The available bandwidth of the egress link is divided among the queues. You configure the queues to be
serviced according to the ratio of WRR weights by using the wrr-queue bandwidth interface
configuration command. The ratio represents the importance (weight) of a queue relative to the other
queues. WRR scheduling prevents low-priority queues from being completely neglected during periods
of high-priority traffic by sending some packets from each queue in turn. The number of packets sent
corresponds to the relative importance of the queue. For example, if one queue has a weight of 3 and
another has a weight of 4, three packets are sent from the first queue for every four that are sent from the
second queue. By using this scheduling, low-priority queues can send packets even though the
high-priority queues are not empty. Queues are selected by the CoS value that is mapped to an egress
queue (CoS-to-egress-queue map) through the wrr-queue cos-map interface configuration command.
All four queues participate in the WRR unless the expedite queue is enabled, in which case the fourth
bandwidth weight is ignored and is not used in the ratio calculation. The expedite queue is a priority
queue, and it is serviced until empty before the other queues are serviced. You enable the expedite queue
by using the priority-queue out interface configuration command.
You can combine the commands described in this section to prioritize traffic by placing packets with
particular DSCPs into certain queues, allocate a larger queue size or service the particular queue more
frequently, and adjust queue thresholds so that packets with lower priorities are dropped. For configuration
information, see the
“Configuring Egress Queues on Gigabit-Capable Ethernet Ports” section on page 29-57
.
Tail Drop
Tail drop is the default congestion-avoidance technique on Gigabit-capable Ethernet ports. With tail
drop, packets are queued until the thresholds are exceeded. Specifically, all packets with DSCPs assigned
to the first threshold are dropped until the threshold is no longer exceeded. However, packets assigned
to the second threshold continue to be queued and sent as long as the second threshold is not exceeded.
You can modify the two tail-drop threshold percentages assigned to the four egress queues by using the
wrr-queue threshold interface configuration command. Each threshold value is a percentage of the total
number of allocated queue descriptors for the queue. The default threshold is 100 percent for
thresholds 1 and 2.
You modify the DSCP-to-threshold map to determine which DSCPs are mapped to which threshold ID
by using the wrr-queue dscp-map interface configuration command. By default, all DSCPs are mapped
to threshold 1, and when this threshold is exceeded, all the packets are dropped.
If you use tail-drop thresholds, you cannot use WRED, and vice versa. If tail drop is disabled, WRED is
automatically enabled with the previous configuration (or the default if it was not previously
configured).