Random Early Detection And Queue Limit; Determining Packet Drop Probability - Cisco 10000 Series Configuration Manual

Quality of service configuration guide

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Controlling Congestion Using Random Early Detection

Random Early Detection and Queue Limit

Random early detection (RED) controls the average congestion level of a class queue and queue limit

controls the instantaneous congestion level of a queue. By using the RED and queue limit mechanisms

together, you can configure a drop policy for a class queue and simultaneously limit the maximum

number of packets allowed to accumulate in the queue.

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

Cisco IOS Release 12.0(25)SX or later releases. In releases prior to Cisco IOS Release 12.0(25)SX, you

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

at the same time.

RED uses three configurable parameters to determine the drop probability of packets: minimum

threshold, maximum threshold, and mark probability denominator. The following describes how RED

determines packet drop probability:

Cisco 10000 Series Router Quality of Service Configuration Guide

RED begins dropping packets when the average queue size is equal to the value of the minimum

RED continues dropping packets when the average queue size is between the minimum and

maximum threshold values.

As the average queue size approaches the maximum threshold, RED uses the mark probability

denominator value to determine the slope (the increase in drop rate). For example, if the

denominator is 512, RED drops one out of every 512 packets when the average queue limit reaches

the maximum threshold.

RED drops all packets when the average queue size is greater than the maximum threshold value.

illustrates how RED determines drop probability.

Determining Drop Probability Using RED

Average Queue Length

Managing Packet Queue Congestion

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