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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 20 Configuring QoS
Understanding QoS
During the queueing and scheduling process, the switch uses egress queues and WRR for congestion
management, and tail drop or WRED algorithms for congestion avoidance on Gigabit-capable Ethernet
ports.
Each Gigabit-capable Ethernet port has four egress queues, one of which can be the egress expedite
queue. You can configure the buffer space allocated to each queue as a ratio of weights by using the
wrr-queue queue-limit interface configuration command, where the relative size differences in the
numbers indicates the relative differences in the queue sizes. To display the absolute value of the queue
size, use the show mls qos interface interface-id statistics privileged EXEC command, and examine
the FreeQ information.
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
“Tail Drop” section on
page 20-13
and
“WRED” section on page 20-14
.
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. 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 not used in the ratio calculation. The expedite queue is a strict-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 20-44
.
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).
