11-20
Cisco 10000 Series Router Quality of Service Configuration Guide
OL-7433-09
Chapter 11 Managing Packet Queue Congestion
Controlling Congestion Using Weighted Random Early Detection
For more information about how WRED uses the minimum and maximum thresholds, and the drop
probability parameters, see the
“Determining Packet Drop Probability” section on page 11-10
.
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.
Note
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.
Note
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.
Table 11-7
Weighted Random Early Detection Default Drop Thresholds
DSCP, Precedence, and
Discard-Class Values
Minimum Threshold
(times the queue size)
Maximum Threshold
(times the queue size)
Drop Probability
All DSCPs
1/4
1/2
1/10
0
1/4
1/2
1/10
1
9/32
1/2
1/10
2
5/16
1/2
1/10
3
11/32
1/2
1/10
4
3/8
1/2
1/10
5
13/32
1/2
1/10
6
7/16
1/2
1/10
7
15/32
1/2
1/10
