Deployment Guide

Using A Configurable Weight for WRED and ECN
The switch supports a user-configurable weight that determines the average queue size used in WRED and Explicit Congestion
Notification (ECN) operation on front-end I/O and backplane interfaces.
By default, the switch uses a weight factor of 0 (instantaneous ECN marking), which results in packet dropping during times of
network congestion based on the configured minimum and maximum WRED thresholds. You can configure different weights for
WRED and ECN operation to finely tune how different types of traffic are handled when a WRED threshold is exceeded.
Benefits of Using a Configurable Weight for WRED with ECN
Using a configurable weight for WRED and ECN allows you to specify how the average queue size is calculated. In WRED, the
average queue size determines when a threshold is exceeded and packets are dropped; in WRED with ECN, the average queue
size determines when packets are marked for later transmission and when the transmission rate is reduced on an interface
during times of network congestion.
For example, in a best-effort network topology that uses WRED with instantaneous ECN, data packets may be transmitted at
a rate in which latency or throughput are not maintained at an effective, optimal level. Packets are dropped when the network
experiences a large traffic load according to the configured WRED thresholds. This best-effort network deployment is not
suitable for applications that are time-sensitive, such as video on demand (VoD) or voice over IP (VoIP) applications.
To resolve the problem of packet loss at times of network congestion, you may need to apply WRED with ECN and more finely
tune packet transmission for certain traffic types. To do so, you can configure the weight used to calculate the average queue
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
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:
Quality of Service (QoS)
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