Administrator Guide

Table Of Contents
If WRED/ECN is enabled on the global service-pool with threshold values and if it is not enabled on the queues, WRED/ECN
are not effective based on global service-pool WRED thresholds. The queue on which traffic is scheduled must have
WRED/ECN settings enabled for WRED to be valid for its traffic.
When WRED is configured on a global service-pool (regardless of whether ECN is configured on the global service-pool),
and one or more queues have WRED enabled and ECN disabled, WRED is effective for the minimum threshold between the
queue threshold and the service-pool threshold.
When WRED is configured on the global service-pool (regardless of whether ECN is configured on the global service-pool),
and one or more queues are enabled with both WRED and ECN, ECN marking takes effect. The packets are ECN marked to
the shared-buffer limits as determined by the shared-ratio for the global service-pool.
WRED/ECN configurations for backplane port queues are applied to all backplane ports and cannot be specified separately on
each backplane port. Also, WRED/ECN is not supported for multicast packets.
The following table describes the WRED and ECN operations performed on a queue and service pool for various WRED with
ECN scenarios. (N/A indicates that a configuration is not applicable. )
Table 82. Scenarios for 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
Disabled Disabled N/A N/A N/A WRED/ECN not
applicable
Enabled Disabled Disabled N/A N/A Queue-based
WRED;
No ECN marking
Enabled N/A Q-T < SP-T
SP-T < Q-T Service-pool-based
WRED;
No ECN marking
Enabled Enabled Disabled N/A N/A Queue-based ECN
marking above
queue threshold.
ECN marking up to
shared buffer limits
of the service-pool
and then packets
are tail dropped.
Enabled N/A Q-T < SP-T
SP-T < Q-T Same as above but
ECN marking starts
above SP-T.
Configuring a Weight for WRED and ECN Operation
You can configure a WRED weight to customize WRED and ECN operation on a front-end or backplane interface. In the
configuration procedure, you must also configure the global service-pools of shared buffer memory that can be accessed by
multiple queues when the minimum guaranteed buffers for a queue are consumed.
1. Configure the weight factor for computation of average-queue size. This weight value applies to front-end and backplane
ports.
QOS-POLICY-OUT mode
Quality of Service (QoS)
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