Reference Guide

size; the average queue size is used to determine when to drop packets with WRED and when to mark packets with ECN when
WRED thresholds are exceeded.
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
On the Z9500, 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 Z9500 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 55. 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
Quality of Service (QoS) 623