Queueing And Scheduling Overview - Cisco Catalyst 2960 series Configuration Manual

Information About QoS

Queueing and Scheduling Overview

The switch has queues at specific points to help prevent congestion.
Figure 56: Egress Queue Location on Switch
The switch supports 4 egress queues by default and there is an option to enable a total of 8 egress queues.
Note
The 8 egress queue configuration is only supported on a standalone switch.
Weighted Tail Drop
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 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).
The following figure 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).
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
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