Concept Guide

You can dene WRED proles and weight on each of the global service-pools for both loss-based and lossless (PFC) service- pools. The
following events occur when you congure 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
eective based on global service-pool WRED thresholds. The queue on which the trac is scheduled must contain WRED/ECN
settings, which are enabled for WRED, to be valid for that trac.
• When WRED is congured on the global service-pool (regardless of whether ECN on global service-pool is congured), and one or
more queues have WRED enabled and ECN disabled, WRED is eective for the minimum of the thresholds between the queue
threshold and the service-pool threshold.
• When WRED is congured on the global service-pool (regardless of whether ECN on global service-pool is congured), and one or
more queues are enabled with both WRED and ECN, ECN marking takes eect. The packets are ECN marked up to shared- buer
limits as determined by the shared-ratio for that global service-pool.
WRED/ECN congurations for the queues that belong to backplane ports are common to all the backplane ports and cannot be specied
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 conguration 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 80. Scenarios of WRED and ECN Conguration
Queue Conguration Service-Pool
Conguration
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
1 1 0 X X Queue-based ECN marking above queue threshold.
ECN marking to shared buer limits of the service-pool and
then packets are tail dropped.
1 X Q-T < SP-T
SP-T < Q-T Same as above but ECN marking starts above SP-T.
Conguring WRED and ECN Attributes
The functionality to congure a weight factor for the WRED and ECN functionality for backplane ports is supported on the platform.
WRED drops packets when the average queue length exceeds the congured threshold value to signify congestion. Explicit Congestion
Notication (ECN) is a capability that enhances WRED by marking the packets instead of causing WRED to drop them when the threshold
value is exceeded. If you congure ECN for WRED, devices employ this functionality of ECN to mark the packets and reduce the rate of
sending packets in a congested, heavily-loaded network.
Quality of Service (QoS)
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