Reference Guide

The following table describes the WRED and ECN operations that occur for various scenarios of WRED
and ECN configuration on the queue and service pool. (X denotes not-applicable in the table, 1 indicates
that the setting is enabled, 0 represents a disabled setting. )
Table 62. Scenarios of WRED and ECN Configuration
Queue
Configuration
Service-Pool
Configuration
WRED Threshold
Relationship
Q threshold = Q-
T,
Service pool
threshold = SP-T
Expected Functionality
WRED ECN WRED ECN
0 0 X X X WRED/ECN not applicable
1 0 0 X X Queue based WRED,
No ECN marking
1 X Q-T < SP-T
SP-T < Q-T SP based WRED,
No ECN marking
1 1 0 X X Queue-based ECN marking above queue
threshold.
ECN marking to shared buffer limits of the
service-pool and then packets are tail dropped.
1 X Q-T < SP-T
SP-T < Q-T Same as above but ECN marking starts above
SP-T.
Configuring WRED and ECN Attributes
The functionality to configure a weight factor for the WRED and ECN functionality for backplane ports is
supported on the Z9000 platform.
WRED drops packets when the average queue length exceeds the configured threshold value to signify
congestion. Explicit Congestion Notification (ECN) is a capability that enhances WRED by marking the
packets instead of causing WRED to drop them when the threshold value is exceeded. If you configure
ECN for WRED, devices employ this functionality of ECN to mark the packets and reduce the rate of
sending packets in a congested, heavily-loaded network.
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Quality of Service (QoS)