• 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 markingtakes 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 thebackplane ports and cannot be specified separately for each backplane port granularity. This behavioroccurs 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 WREDand ECN configuration on the queue and service pool. (X denotes not-applicable in the table, 1 indicatesthat the setting is enabled, 0 represents a disabled setting. )Table 42. Scenarios of WRED and ECN ConfigurationQueueConfiguration Service-PoolConfiguration WRED ThresholdRelationshipQ threshold = Q-T,Service poolthreshold = SP-TExpected FunctionalityWRED ECN WRED ECN0 0 X X X WRED/ECN not applicable1 0 0 X X Queue based WRED,No ECN marking1 X Q-T < SP-TSP-T < Q-T SP based WRED,No ECN marking1 1 0 X X Queue-based ECN marking above queuethreshold.ECN marking to shared buffer limits of theservice-pool and then packets are tail dropped.1 X Q-T < SP-TSP-T < Q-T Same as above but ECN marking starts aboveSP-T.Configuring WRED and ECN AttributesThe functionality to configure a weight factor for the WRED and ECN functionality for backplane ports issupported on the Z9000 platform. The functionality to configure a weight for WRED and ECNfunctionality for front-end ports is supported on the Z9000 platform. A global buffer pool that is a sharedbuffer pool accessed by multiple queues when the minimum guaranteed buffers for the queue areconsumed can be configured on the Z9000 platform.WRED drops packets when the average queue length exceeds the configured threshold value to signifycongestion. Explicit Congestion Notification (ECN) is a capability that enhances WRED by marking thepackets instead of causing WRED to drop them when the threshold value is exceeded. If you configureQuality of Service (QoS) 665
