Users Guide

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 buer thresholds for each color-coding of the packets. You can congure maximum drop rate
percentage of yellow and green proles. 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 congure global service pools that are shared buer pools accessed by
multiple queues when the minimum guaranteed buers for the queue are consumed. S4820T platform support four global service-
pools in the egress direction. Two service pools are used– one for loss-based queues and the other for lossless (priority-based ow
control (PFC)) queues. You can enable WRED and ECN conguration on the global service-pools.
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 65. 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
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Quality of Service (QoS)