Wred And Queue Limit - Cisco 10000 Series Configuration Manual

Quality of service configuration guide

Hide thumbs Also See for 10000 Series:

Table of Contents

Controlling Congestion Using Weighted Random Early Detection

DSCP, Precedence, and

Discard-Class Values

For more information about how WRED uses the minimum and maximum thresholds, and the drop

probability parameters, see the

WRED and Queue Limit

To help you manage packet delay due to congestion, you can configure weighted random early detection

(WRED) on a class queue and customize the size of the queue using the queue-limit command. This

allows you to establish a drop policy for a traffic class and simultaneously limit the maximum number

of packets allowed to accumulate in the class queue.

WRED controls the average length of a queue and queue-limit controls the maximum number of packets

that can be waiting for transmission at any time. Together, these mechanisms limit packet delay.

To simultaneously configure WRED and queue limit for a class in a policy map, the router must be

running Cisco IOS Release 12.3(7)XI or later releases. In releases prior to Cisco IOS Release 12.3(7)XI,

you can configure either the random-detect command or the queue-limit command, but not both

commands at the same time.

If you do not specify a queue limit, the router uses the default queue limit of two times the largest

maximum threshold, rounded to the nearest power of 2.

To modify the queue length, always use the queue-limit command instead of the max-threshold

parameter of the random-detect command. Modifying the max-threshold parameter does not

necessarily change the queue limit. When you increase the max-threshold parameter, WRED adjusts the

queue length to be no less than the max-threshold value. However, when you reduce the max-threshold

parameter, WRED does not change the queue length.

Cisco 10000 Series Router Quality of Service Configuration Guide

Weighted Random Early Detection Default Drop Thresholds

Minimum Threshold

(times the queue size)

"Determining Packet Drop Probability" section on page

Managing Packet Queue Congestion

Maximum Threshold

(times the queue size)

Drop Probability

Table of Contents