Addendum

such cases, you can use ECN in conjunction with WRED to resolve this problem of dropping of packets
under congested conditions.
Using ECN, instead of entirely dropping the packets when the network experiences excessive traffic, the
packets are marked for transmission at a later time after the network recovers from the heavy traffic state
to optimal load. In this manner, enhanced performance and throughput is achieved. Also, the devices can
respond to congestion before a queue overflows and packets are dropped, enabling improved queue
management.
When a packet reaches the device with ECN enabled for WRED, the average queue size is computed. To
measure the average queue size, a weight factor is used. This weight factor is user-configurable. You can
use the wred weight number command to configure the weight for the WRED average queue size. If the
average queue size is less than the minimum threshold of WRED, the received packet is queued. If the
average queue size is more than the maximum threshold of WRED, the packet is dropped. If the average
queue size is between the minimum and maximum threshold values, the decision to drop or queue the
packet is taken based on the packet drop probability. The probability that a packet is dropped depends on
the minimum threshold, maximum threshold, and mark probability denominator. The rate of packet drop
increases in a proportional way as the average queue size increases, until the average queue size reaches
the maximum threshold value. The mark probability value is the number of packets dropped when the
average queue size reaches the maximum threshold value.
The average queue size is computed using the preceding average 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 time
iteration at which average queue size is calculated, and N is the weight factor.
The weight factor is set to zero by default, which causes the same behavior as dropping of packets by
WRED during network loads or also called instantaneous ECN marking. In a topology in which congestion
of the network varies over time, you can specify a weight to enable a smooth, seamless averaging of
packets to handle bursty nature of the packets based on the previous time sampling performed. You can
specify the weight parameter for front-end and backplane ports separately in the range of 0 through 15.
You can enable WRED and ECN capabilities per queue for granularity. You can disable these
functionalities per queue, and you can also specify the minimum and maximum buffer thresholds for
each color-coding of the packets. You can configure maximum drop rate percentage or yellow and
green profiles. You can set up these parameters for both front-end and backplane ports.
Global Service Pools With WRED and ECN Settings
A global buffer pool that is a shared buffer pool accessed by multiple queues when the minimum
guaranteed buffers for the queue are consumed can be configured on the S6000 and Z9000 platforms.
Support for global service pools is now available. You can configure global service pools that are shared
buffer pools accessed by multiple queues when the minimum guaranteed buffers for the queue are
consumed. S4810, S4820T, S6000, and Z9000 platforms support four global service-pools in the egress
direction. Two service pools are used– one for lossy queues and the other for lossless (priority-based
flow control (PFC)) queues. You can enable WRED and ECN configuration on the global service-pools.
You can define WRED profiles and weight on each of the global service-pools for both lossy and lossless
(PFC) service- pools. The following events occur when you configure WRED and ECN on global service
pools:
• 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 contain WRED/ECN settings enabled for WRED to be valid for that
traffic.
Quality of Service (QoS)
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