26-14
Catalyst 3750 Metro Switch Software Configuration Guide
78-15870-01
Chapter 26 Configuring QoS
Understanding QoS
Figure 26-6 WTD and Queue Operation
For more information, see the “Mapping DSCP or CoS Values to an Ingress Queue and Setting WTD
Thresholds” section on page 26-65, the “Allocating Buffer Space to and Setting WTD Thresholds for an
Egress Queue-Set” section on page 26-69, and the “Mapping DSCP or CoS Values to an Egress
Queue-Set and to a Threshold ID” section on page 26-71.
SRR Shaping and Sharing
The ingress queues and egress queue-sets are serviced by SRR, which controls the rate at which packets
are sent. On the ingress queues, SRR sends packets to the internal ring. On the egress queue-sets, SRR
sends packets to a standard port.
You can configure SRR on the egress queue-sets for sharing or for shaping. However, for ingress queues,
sharing is the default mode and is the only mode supported.
In shaped mode, the queues are guaranteed a percentage of the bandwidth, and they are rate-limited to
that amount. Shaped traffic does not use more than the allocated bandwidth even if the link is idle.
Shaping provides a more even flow of traffic over time and reduces the peaks and valleys of bursty
traffic. With shaping, the absolute value of each weight is used to compute the bandwidth available for
the queues.
In shared mode, the queues share the bandwidth among them according to the configured weights. The
bandwidth is guaranteed at this level but not limited to it. For example, if a queue is empty and no longer
requires a share of the link, the remaining queues can expand into the unused bandwidth and share it
among them. With sharing, the ratio of the weights controls the frequency of dequeuing; the absolute
values are meaningless.
For more information, see the “Allocating Bandwidth Between the Ingress Queues” section on
page 26-67, the “Configuring SRR Shaped Weights on an Egress Queue-Set” section on page 26-72, and
the “Configuring SRR Shared Weights on an Egress Queue-Set” section on page 26-73.
CoS 6-7
100%
60%
40%
1000
600
400
0
CoS 4-5
CoS 0-3
86692
To Next Page
Loading...
