Cisco Catalyst 2960 Series, Configuration Manual

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These percentages indicate that up to 400 frames can be queued at the 40-percent threshold, up to 600 frames
at the 60-percent threshold, and up to 1000 frames at the 100-percent threshold.
Figure 57: WTD and Queue Operation
In the example, CoS values 6 and 7 have a greater importance than the other CoS values, and they are assigned
to the 100-percent drop threshold (queue-full state). CoS values 4 and 5 are assigned to the 60-percent threshold,
and CoS values 0 to 3 are assigned to the 40-percent threshold.
Suppose the queue is already filled with 600 frames, and a new frame arrives. It contains CoS values 4 and
5 and is subjected to the 60-percent threshold. If this frame is added to the queue, the threshold will be exceeded,
so the switch drops it.
Related Topics
Mapping DSCP or CoS Values to an Ingress Queue and Setting WTD Thresholds, on page 612
Allocating Buffer Space to and Setting WTD Thresholds for an Egress Queue-Set, on page 618
Mapping DSCP or CoS Values to an Egress Queue and to a Threshold ID, on page 621
WTD Thresholds, on page 558
Queues and WTD Thresholds, on page 562
SRR Shaping and Sharing
Egress queues are serviced by shaped round robin (SRR), which controls the rate at which packets are sent.
On the egress queues, SRR sends packets to the egress port.
You can configure SRR on egress queues for sharing or for shaping.
In shaped mode, the egress 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. Shaping and sharing is configured per interface. Each interface can be uniquely configured.
Related Topics
Ingress Port Activity
Allocating Bandwidth Between the Ingress Queues, on page 616
Configuring SRR Shaped Weights on Egress Queues, on page 623
Configuring SRR Shared Weights on Egress Queues, on page 625
Shaped or Shared Mode, on page 562
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
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Information About QoS