Srr Shaping And Sharing - Cisco Catalyst 3750 Software Configuration Manual

Understanding QoS
Figure 26-6 WTD and Queue Operation
CoS 6-7
CoS 4-5
CoS 0-3
For more information, see the
Thresholds" section on page
Egress Queue-Set" section on page
Queue-Set and to a Threshold ID" section on page

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
page
the
Catalyst 3750 Metro Switch Software Configuration Guide
26-14
100%
60%
40%
26-65, the
26-67, the "Configuring SRR Shaped Weights on an Egress Queue-Set" section on page
"Configuring SRR Shared Weights on an Egress Queue-Set" section on page
1000
600
400
0
"Mapping DSCP or CoS Values to an Ingress Queue and Setting WTD
"Allocating Buffer Space to and Setting WTD Thresholds for an
26-69, and the "Mapping DSCP or CoS Values to an Egress
26-71.
"Allocating Bandwidth Between the Ingress Queues" section on
Chapter 26 Configuring QoS
26-72, and
26-73.
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