Configuring Static ECN

If the device does not support AI ECN and dynamic ECN, you can configure the static ECN function and set the static ECN threshold for a specified queue. To configure static ECN, run the following commands:

<HUAWEI> system-view
[~HUAWEI] qos ecn   //Enable ECN globally. This step is mandatory only on the CE12800.
[~HUAWEI] drop-profile drop1   //Create a WRED drop profile and enter the WRED drop profile view.
[*HUAWEI-drop-drop1] ecn buffer-size low-limit 7000 high-limit 15000 discard-percentage 30   //Specify the static ECN parameters: absolute values of upper and lower drop thresholds, and maximum drop probability.

Recommended settings:

• Run the ecn command based on the expected round trip time (RTT) of RoCEv2 packets. The bandwidth-delay product (BDP) can be calculated based on the expected RTT. Generally, the upper ECN threshold is two to three times the BDP. The BDP is calculated according to the following formula:

  BDP = RTT x Interface bandwidth

  Typically, the expected RTT is 20 μs by default. For a spine node with the interface bandwidth of 100 Gbit/s, the BDP is calculated as follows:

  BDP = 20 μs x 100 Gbit/s = 2000 Kbits = 250 KBytes

  In this case, the upper ECN threshold of the spine node can be set to 2 to 3 times of 250 KBytes. Table 2-12 lists the recommended configurations shown in most scenarios of AI Fabric.

  Table 2-12 Recommended ECN configuration for scenarios with an expected RTT of 20 μs

  Device Model	Application Scenario	Interface Bandwidth	Lower ECN Threshold (Marking by Probability)	Upper ECN Threshold (Marking All)	Maximum Drop Probability (Maximum Marking Probability)	Remarks
  CE6865-48S8CQ-EI, CE8850-64CQ-EI, CE8861-4C-EI	Delay-sensitive but not throughput-sensitive	25GE	7 KB	150 KB	30%	These parameters are set based on the expected RTT of 20 μs.
  		100GE	7 KB	500 KB	30%
  	Throughput-sensitive but not delay-sensitive	25GE	7 KB	200 KB	10%
  		100GE	7 KB	750 KB	10%
  CloudEngine 16800	All	100GE	7 KB	18 MB	100%	-

• The maximum drop probability configured using the discard-percentage discard-percentage-value command is not the actual maximum value. Table 2-13 lists the mapping between the configured maximum drop probability and the actual maximum value.
  Table 2-13 Mapping between the configured maximum drop probability and the actual maximum value

  Configured Maximum Drop Probability	Actual Maximum Value
  1 to 9	Same as the configured maximum drop probability
  10 to 24	10
  25 to 49	25
  50 to 74	50
  75 to 99	75
  100	100