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Catalyst 3550 Multilayer Switch Software Configuration Guide
78-11194-09
Chapter 29 Configuring QoS
Understanding QoS
Figure 29-5 Queueing and Scheduling Flowchart for Gigabit-Capable Ethernet Ports
Note If the expedite queue is enabled, WRR services it until it is empty before servicing the other three
queues.
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 show 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.
46978
T1 and T2 thresholds
Queue size
Queue number
No
Ye s
Read CoS value and the
CoS-to-queue map.
Determine high and low
threshold of the queue,
and determine the queue size.
Are thresholds
being exceeded?
Put packet in the specified
queue and service the
queue according to WRR.
Determine which DSCPs are
mapped to each threshold.
Read the DSCP-to-threshold
map.
Drop packet.
Start
Done
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