Configuring QoS OmniSwitCongestion Avoidance
OmniSwitch AOS Release 8 Network Configuration Guide December 2017 page 26-19
OmniSwitCongestion Avoidance
Congestion avoidance mechanisms monitor queues to provide early detection and notification of potential
queue congestion. If necessary, such mechanisms may even strategically drop low priority (non-
conforming) packets to prevent congestion. Dropping packets signals the packet source to decrease the
transmission rate, thus preventing the queue from overflowing.
The OmniSwitch uses Weighted Random Early Detection (WRED) for admission control and congestion
avoidance. WRED continuously monitors the average queue length over time. When the average length
exceeds specific queue thresholds, WRED begins to randomly drop packets with a specific probability.
WRED uses the drop precedence (color) of the packet to determine which packets to drop. A packet is
color marked during the QoS classification process to indicate a drop precedence for the packet.
• Green = Committed
• Yellow = Conformed
• Red = Exceeded
Color marking techniques supported include Single-Rate Tri-Color Marking (srTCM) and Two-Rate Tri-
Color Marking (trTCM). TCM is applied to ingress traffic using a QoS policy rule (see “Configuring Tri-
Color Marking” on page 26-24 for more information). Note that all packets that are not marked with a
specific color are treated as green (committed) packets.
WRED Profiles
This implementation of WRED uses a drop profile to apply active queue management to the output
queues. The profile specifies queue thresholds (as a percentage of maximum queue size) and drop
probability values for green, yellow, and red traffic. The thresholds specify a linear relationship between
average queue length and drop probability.
The WRED profile (WRP) defines the following drop precedence parameters for packets marked green,
yellow, and red:
• The minimum queue length threshold (percentage of queue size).
• The maximum queue length threshold (percentage of queue size).
• The drop probability (percentage of packets to be dropped).
• The queue gain (a numeric value used to determine the instantaneous average queue length).
The OmniSwitch 6900 supports the use of one WRED profile (WRP 1), which is applied to TCP traffic
when the profile is enabled for the QSet instance. By default, WRP 1 is associated with each QSet profile
and applied to each unicast queue within the QSet instance associated with the QSet profile.
Note. The OmniSwitch 6900 supports WRED only on TCP traffic.WRED is not supported on any other
OmniSwitch platforms.
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