About WRED tables

The switch supports queue-based WRED tables. You can configure separate drop parameters for different queues. When congestion occurs, packets of a queue are randomly dropped based on the drop parameters of the queue.

Determine the following parameters before configuring WRED:

• Upper threshold and lower threshold—When the average queue size is smaller than the lower threshold, packets are not dropped. When the average queue size is between the lower threshold and the upper threshold, the packets are dropped at random. The longer the queue, the higher the drop probability. When the average queue size exceeds the upper threshold, subsequent packets are dropped.

• Drop precedence—A parameter used for packet drop. The value 0 corresponds to green packets, the value 1 corresponds to yellow packets, and the value 2 corresponds to red packets. Red packets are dropped preferentially.

• Exponent for average queue size calculation—The greater the exponent, the less sensitive the average queue size is to real-time queue size changes. The formula for calculating the average queue size is:

  Average queue size = ( previous average queue size x (1 – 2^–n) ) + (current queue size x 2^–n), where n is the exponent.

• Drop probability—This parameter is used when a WRED table is configured. The drop probability is expressed in percentage. The greater the percentage value, the greater the drop probability.