Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.112.0P1 - Page 728
Configuring WRED and ECN Attributes
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Table 79. Scenarios of WRED and ECN Configuration Queue Configuration Service-Pool Configuration WRED Threshold Expected Functionality Relationship Q threshold = Q-T, Service pool threshold = SP-T WRED ECN WRED ECN 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 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 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. To configure the weight factor for WRED and ECN capabilities, global buffer pools for multiple queues, and associating a service class with ECN marking, perform the following: 1 Configure the weight factor for the computation of average-queue size. This weight value applies to front-end ports. QOS-POLICY-OUT mode Dell(conf-qos-policy-out)#wred-profile weight number 2 Configure a WRED profile, and specify the threshold and maximum drop rate. WRED mode Dell(conf-wred) #wred-profile thresh-1 Dell(conf-wred) #threshold min 100 max 200 max-drop-rate 40 3 Configure another WRED profile, and specify the threshold and maximum drop rate. WRED mode Dell(conf-wred) #wred-profile thresh-2 Dell(conf-wred) #threshold min 300 max 400 max-drop-rate 80 4 Create a global buffer pool that is a shared buffer pool accessed by multiple queues when the minimum guaranteed buffers for the queue are consumed. S4820T platform supports four global service-pools in the egress direction. 728 Quality of Service (QoS)
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