Queueing And Scheduling Overview; Weighted Tail Drop - Cisco Catalyst 3120 Software Manual

Catalyst blade switch for hp

Hide thumbs Also See for Catalyst 3120:

Table of Contents

Configuring QoS

Queueing and Scheduling Overview

The switch has queues at specific points to help prevent congestion as shown in

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.

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).

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.

shows an example of WTD operating on a queue whose size is 1000 frames. Three drop

Cisco Catalyst Blade Switch 3120 for HP Software Configuration Guide

Understanding QoS

Table of Contents