Administrator Guide

The user-configurable weight in WRED and ECN provides better control in how the switch responds to congestion before a queue
overflows and packets are dropped or delayed. Using a configurable weight for WRED and ECN allows you to customize network
performance and throughput.
Setting Average Queue Size using a Weight
You can configure the weight factor that determines the average queue size for WRED and ECN packet handling by using the wred
weight command.
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. )
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
Quality of Service (QoS) 761