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Cisco Catalyst Blade Switch 3130 and 3032 for Dell Software Configuration Guide
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Chapter 36 Configuring QoS
Understanding QoS
Queueing and Scheduling Overview
The switch has queues at specific points to help prevent congestion as shown in
.
Figure 36-6
Ingress and Egress Queue Location
Because the total inbound bandwidth of all ports can exceed the bandwidth of the stack or internal ring,
ingress queues are located after the packet is classified, policed, and marked and before packets are
forwarded into the switch fabric. Because multiple ingress ports can simultaneously send packets to an
egress port and cause congestion, outbound queues are located after the stack or internal ring.
Weighted Tail Drop
Both the ingress and egress queues use an enhanced version of the tail-drop congestion-avoidance
mechanism called weighted tail drop (WTD). WTD is implemented on queues to manage the queue
lengths and to provide drop precedences for different traffic classifications.
As a frame is enqueued to a particular queue, WTD uses the frame’s assigned QoS label to subject it to
different thresholds. If the threshold is exceeded for that QoS label (the space available in the destination
queue is less than the size of the frame), the switch drops the frame.
Each queue has three threshold values. The QOS label is determines which of the three threshold values
is subjected to the frame. Of the three thresholds, two are configurable (explicit) and one is not (implicit).
shows an example of WTD operating on a queue whose size is 1000 frames. Three drop
percentages are configured: 40 percent (400 frames), 60 percent (600 frames), and 100 percent (1000
frames). These percentages mean 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.
In this 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.
Marker
Policer
Marker
Policer
Marker
Ingress
queues
Stack ring
Egress
queues
Policer
Marker
Policer
Classify
Traffic
SRR
SRR
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