Controlling Congestion Using Random Early Detection; Feature History For Random Early Detection - Cisco 10000 Series Configuration Manual
Quality of service configuration guide
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Chapter 11
Managing Packet Queue Congestion
Controlling Congestion Using Random Early Detection
Random early detection (RED) is an alternative mechanism for controlling congestion of Layer 3
queues. Unlike the tail drop mechanism, RED implements a proactive queuing strategy that controls
congestion before a queue reaches its queue limit.
RED randomly discards packets. If the source host is using Transmission Control Protocol (TCP), the
host detects that packets are dropped and decreases its transmission rate until all of the packets reach
their destinations, indicating that the congestion is clear. The host can then resume its original
transmission rate.
You can configure a drop policy for RED that is based on one of the following. Each queue on the router
can have only one type of RED drop policy.
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•
Feature History for Random Early Detection
Cisco IOS Release
Release 12.0(17)SL
Release 12.0(22)S
Release 12.0(25)SX
Release 12.2(16)BX
Release 12.2(28)SB
Release 12.2(31)SB2
OL-7433-09
IP precedence-based RED—Configures a drop policy for RED based on an IP precedence level.
Valid values are from 0 to 7, where 0 typically represents low priority traffic that can be aggressively
managed (dropped) and 7 represents high priority traffic. Traffic at a low precedence level typically
has a higher drop probability. When RED drops packets, source hosts using TCP detect the drops
and slow the transmission of packets.
DSCP-based RED—Configures a drop policy for RED based on a differentiated services code point
(DSCP) value. When configured, the router randomly drops packets with the specified DSCP value,
according to the RED thresholds you configure. For the PRE1, DSCP-based RED supports one
default drop profile per class, three assured forwarding (AF) drop profiles per class, and four
non-AF drop profiles per policy map.
Description
The random early detection feature was introduced on the
router.
This feature was enhanced to allow you to configure RED
based on a differentiated services code point (DSCP).
This feature was enhanced to allow you to simultaneously
configure the random-detect command and the
queue-limit for the same class queue.
This feature was introduced on the PRE2.
This feature was integrated in Cisco IOS
Release 12.2(28)SB for the PRE2.
This feature was introduced on the PRE3.
Controlling Congestion Using Random Early Detection
Cisco 10000 Series Router Quality of Service Configuration Guide
Required PRE
PRE1
PRE1
PRE1
PRE2
PRE2
PRE3
11-9
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