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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
•
Frame Relay DE WRED—The discard eligibility (DE) bit in the address field of a frame relay frame
is used to prioritize the discarding of frames in congested frame relay networks. The frame relay DE
bit has only one bit and therefore only has two settings, 0 or 1. If congestion occurs in a frame relay
network, frames with the DE bit set at 1 are discarded before frames with the DE bit set at 0.
Therefore, important traffic should have the DE bit set at 0 while less important traffic should be
forwarded with the DE bit set at 1.
You can also configure WRED to ignore the IP precedence, DSCP, or discard-class when making drop
decisions. As a result, the router implements non-weighted random early detection (RED) behavior when
deciding which packets to drop.
Note
The PRE2 uses sampled RED and the PRE3 uses per-packet RED.
Feature History for Weighted Random Early Detection
Cisco IOS Release
Description
Required PRE
Release 12.0(17)SL
The weighted random early detection feature was
introduced on the router.
PRE1
Release 12.0(22)S
This feature
was enhanced to allow you to configure
WRED based on a differentiated services code point
(DSCP).
PRE1
Release 12.0(25)SX
This feature was enhanced to allow you to simultaneously
configure the
random-detect
command and the
queue-limit
for the same class queue.
PRE1
Release 12.2(16)BX
This feature was introduced on the PRE2 and uses sampled
RED.
PRE2
Release 12.3(7)XI
This feature was enhanced on the PRE2 to:
•
Enable the configuration of eight unique drop
precedence levels for one queue instead of four levels
•
Allow the simultaneous configuration of both the
random-detect
and
queue-limit
commands for a
class queue
•
Support discard-class-based WRED
•
Maintain separate WRED drop statistics for each IP
precedence, DSCP value, and discard-class
PRE2
Release 12.2(28)SB
This feature was integrated in Cisco IOS
Release 12.2(28)SB for the PRE2.
PRE2
Release 12.2(31)SB2
This feature was introduced on the PRE3 and uses
per-packet RED. This feature is DiffServ-compliant on the
PRE3.
PRE3
Release 12.2(33)SB
This feature was enhanced to support ATM cell loss
priority-based WRED on the PRE2, PRE3 and PRE4.
PRE2, PRE3,
PRE4
