Users Guide

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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.
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Quality of Service (QoS)