Weighted Tail Drop - Cisco Catalyst 2960 Software Configuration Manual
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Chapter 33
Configuring QoS
Because the total inbound bandwidth of all ports can exceed the bandwidth of the stack 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 ring.
Weighted Tail Drop
To use WTD, the switch must be running the LAN Base image.
Note
Catalyst 2960-S switches do not support ingress queueing.
Note
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).
Figure 33-6
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.
Figure 33-6
CoS 6-7
CoS 4-5
CoS 0-3
For more information, see the
Thresholds" section on page
Egress Queue-Set" section on page
and to a Threshold ID" section on page
OL-26520-01
shows an example of WTD operating on a queue whose size is 1000 frames. Three drop
WTD and Queue Operation
100%
1000
60%
600
40%
400
0
"Mapping DSCP or CoS Values to an Ingress Queue and Setting WTD
33-71, the
33-76, and the
Catalyst 2960 and 2960-S Switches Software Configuration Guide, Release 15.0(1)SE
"Allocating Buffer Space to and Setting WTD Thresholds for an
"Mapping DSCP or CoS Values to an Egress Queue
33-78.
Understanding QoS
33-13
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