Dell PowerSwitch S4820T 9.50.1 Configuration Guide for the S4820T System - Page 758
Configuring WRED and ECN Attributes, Queue, Configuration, Service-Pool, WRED Threshold, Relationship
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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 servicepool 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 sharedratio 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 60. Scenarios of WRED and ECN Configuration Queue Configuration WRED ECN Service-Pool Configuration WRED ECN WRED Threshold Relationship Q threshold = QT, Service pool threshold = SP-T Expected Functionality 0 0 X X X WRED/ECN not applicable 1 0 0 1 X X X Q-T < SP-T Queue based WRED, No ECN marking SP-T < Q-T SP based WRED, No ECN marking 1 1 0 1 X X X Q-T < SP-T Queue-based ECN marking above queue threshold. ECN marking to shared buffer limits of the service-pool and then packets are tail dropped. SP-T < Q-T Same as above but ECN marking starts above SP-T. Configuring WRED and ECN Attributes The functionality to configure a weight factor for the WRED and ECN functionality for backplane ports is supported on the Z9000 platform. WRED drops packets when the average queue length exceeds the configured threshold value to signify congestion. Explicit Congestion Notification (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 configure 758 Quality of Service (QoS)
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