Configuring Modular QoS Congestion Avoidance on Cisco ASR 9000 Series Routers
Prerequisites for Configuring Modular QoS Congestion Avoidance on Cisco ASR 9000 Series Routers
QC-74
Cisco ASR 9000 Series Aggregation Services Router Modular Quality of Service Configuration Guide
OL-23108-02
• How to Configure Modular QoS Congestion Avoidance on Cisco ASR 9000 Series Routers, page 76
• Additional References, page 86
Prerequisites for Configuring Modular QoS Congestion
Avoidance on Cisco
ASR 9000 Series Routers
The following prerequisite is required for configuring QoS congestion avoidance on your network:
You must be in a user group associated with a task group that includes the proper task IDs. The command
reference guides include the task IDs required for each command. If you suspect user group assignment
is preventing you from using a command, contact your AAA administrator for assistance.
Information About Configuring Modular QoS Congestion
Avoidance on Cisco
ASR 9000 Series Routers
To configure QoS congestion avoidance techniques in this document you must understand the following
concepts:
• Random Early Detection and TCP, page 74
• Weighted Random Early Detection for Preferential Traffic Handling, page 74
• Tail Drop and the FIFO Queue, page 76
Random Early Detection and TCP
The RED congestion avoidance technique takes advantage of the congestion control mechanism of TCP.
By randomly dropping packets prior to periods of high congestion, RED tells the packet source to
decrease its transmission rate. Assuming the packet source is using TCP, it decreases its transmission
rate until all packets reach their destination, indicating that the congestion is cleared. You can use RED
as a way to cause TCP to slow transmission of packets. TCP not only pauses, but it also restarts quickly
and adapts its transmission rate to the rate that the network can support.
RED distributes losses in time and maintains normally low queue depth while absorbing traffic bursts.
When enabled on an interface, RED begins dropping packets when congestion occurs at a rate you select
during configuration.
Weighted Random Early Detection for Preferential Traffic Handling
WRED provides the ability to define multiple RED profiles within a single class, based on certain match
criteria (DSCP, discard class and so on), so that different drop precedences can be configured based on
the relative importance of packets. WRED can selectively discard lower priority traffic when the
interface begins to get congested and provide differentiated performance characteristics for different
classes of service. You can configure WRED to ignore IP precedence when making drop decisions so
that nonweighted RED behavior is achieved.
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