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Catalyst 3750 SwitchSoftware Configuration Guide
OL-8550-09
Chapter35 Configuring QoS
Understanding QoS
modify it. You map a port to queue-set by using the queue-set qset-id interface configuration command.
Modify the queue-set configuration to change the WTD threshold percentages. For more information
about how WTD works, see the “Weighted Tail Drop” section on page35-14.

Shaped or Shared Mode

SRR services each queue-set in shared or shaped mode. You assign shared or shaped weights to the port
by using the srr-queue bandwidth share weight1 weight2 weight3 weight4 or the srr-queue bandwidth
shape weight1 weight2 weight3 weight4 interface configuration commands. For an explanation of the
differences between shaping and sharing, see the “SRR Shaping and Sharing” section on page35-15.
Note You cannot assign shaped weights on 10-Gigabit interfaces.
The buffer allocation together with the SRR weight ratios control how much data can be buffered and
sent before packets are dropped. The weight ratio is the ratio of the frequency in which the SRR
scheduler sends packets from each queue.
All four queues participate in the SRR unless the expedite queue is enabled, in which case the first
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 or CoSs into certain queues, by allocating a large queue size or by servicing the queue
more frequently, and by adjusting queue thresholds so that packets with lower priorities are dropped. For
configuration information, see the “Configuring Egress Queue Characteristics” section on page 35-77.
Note The egress queue default settings are suitable for most situations. You should change them only when
you have a thorough understanding of the egress queues and if these settings do not meet your QoS
solution.
Packet Modification
A packet is classified, policed, and queued to provide QoS. Packet modifications can occur during this
process:
•For IP and non-IP packets, classification involves assigning a QoS label to a packet based on the
DSCP or CoS of the received packet. However, the packet is not modified at this stage; only an
indication of the assigned DSCP or CoS value is carried along. The reason for this is that QoS
classification and forwarding lookups occur in parallel, and it is possible that the packet is forwarded
with its original DSCP to the CPU where it is again processed through software.
•During policing, IP and non-IP packets can have another DSCP assigned to them (if they are out of
profile and the policer specifies a markdown DSCP). Once again, the DSCP in the packet is not
modified, but an indication of the marked-down value is carried along. For IP packets, the packet
modification occurs at a later stage; for non-IP packets the DSCP is converted to CoS and used for
queueing and scheduling decisions.
•Depending on the QoS label assigned to a frame and the mutation chosen, the DSCP and CoS values
of the frame are rewritten. If you do not configure the mutation map and if you configure the port to
trust the DSCP of the incoming frame, the DSCP value in the frame is not changed, but the CoS is