Developers Guide

Using ECN, the packets are marked for transmission at a later time after the network recovers from the heavy traffic state
to an optimal load. In this manner, enhanced performance and throughput are 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. The mark probability value is the number of packets dropped when
the average queue size reaches the maximum threshold value.
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 the sudden overload of 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 functionality 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 of 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
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. Two service pools are used– one
for loss-based 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 loss-based 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 the traffic is scheduled must contain
WRED/ECN settings, which are enabled for WRED, to be valid for that traffic.
● When WRED is configured on the global service-pool (regardless of whether ECN on global service-pool is configured), and
one or more queues have WRED enabled and ECN disabled, WRED is effective for the minimum of the thresholds between
the queue threshold and the service-pool threshold.
● When WRED is configured on the global service-pool (regardless of whether ECN on global service-pool is configured), and
one or more queues are enabled with both WRED and ECN, ECN marking takes effect. The packets are ECN marked up to
shared- buffer limits as determined by the shared-ratio for that global service-pool.
WRED/ECN configurations for the queues that belong to backplane ports are common to all the backplane ports and cannot be
specified separately for each backplane port granularity. This behavior occurs to prevent system-level complexities in enabling
this support for backplane ports. Also, WRED/ECN is not supported for multicast packets.
The following table describes the WRED and ECN operations that occur for various scenarios of WRED and ECN configuration
on the queue and service pool. (X denotes not-applicable in the table, 1 indicates that the setting is enabled, 0 represents a
disabled setting. )
Table 83. Scenarios of 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
0 0 X X X WRED/ECN not applicable
1 0 0 X X Queue based WRED,
No ECN marking
1 X Q-T < SP-T
SP-T < Q-T SP based WRED,
No ECN marking
Quality of Service (QoS) 717