Administrator Guide

Global Service-Pools for WRED with ECN
You can enable WRED with ECN to work with global service-pools. Global service pools that function as shared buffers are
accessed by multiple queues when the minimum guaranteed buffers for a queue are consumed. The switch supports four global
service-pools in the egress direction.
Two types of service-pools are used: one for lossy queues and the other for lossless (priority-based flow control (PFC)) queues.
NOTE: Service pool 1 for lossless queues is not supported in software releases that do not support PFC.
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 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
802 Quality of Service (QoS)