Users Guide

You can define WRED profiles and a weight on global service-pools for both lossy and lossless (PFC)
service-pools. The following events occur when you configure WRED with ECN on a global service-pool:
• 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 76. 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
Quality of Service (QoS)
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