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The average queue size is computed using the last calculated average-queue size and the current queue size.
The following is the formula to calculate the average queue size: average-queue-size (t+1) = average-queue-
size (t) + (current-queue-length - average-queue-size (t))/2^N
where t is the time or the current instant at which average queue size is measured, t+1 is the next calculation
of the average queue size, and N is the weight factor.
In a topology in which network congestion varies over time, you can specify a weight to enable a smooth,
seamless averaging of packets to handle the bursty nature of packets based on the previous time sampling
performed. You can specify a weight value for front-end and backplane ports separately. The range of weight
values is from 0 to 15.
You can enable WRED with ECN capabilities per queue to fine-tune packet transmission. You can disable
WRED with ECN per queue while configuring the minimum and maximum buffer thresholds for each WRED
color-coded profile. You can configure the maximum drop-rate percentage for yellow and green profiles.
You can configure these parameters for both front-end and backplane ports.
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. )
Quality of Service (QoS)
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Troubleshooting
