WRED Thresholds confusion

[AhsanMohiuddin]

Hi,

MLS switches (e.g. 3550) have an interface command "wrr-queue random-detect {max-threshold|min-threshold} queue-id threshold1 threshold2"

The threshold1 and threshold2 refer to WRED thresholds e.g. if threshold1 is set to 40% and threshold2 to 80%, WRED will start dropping SOME packets when queue is 40% full, and ALL packets when 80% full.

Question is, what does max-threshold and min-threshold refer to?

[ariela · Accepted solution]

http://www.cisco.com/en/US/tech/tk389/tk813/technologies_tech_note09186a00801558cb.shtml

- Min threshold: Represents a threshold within a queue. No packets are dropped below this threshold.

- Maximum (max) threshold: Represents another threshold within a queue. All packets are dropped above the max threshold.

- Slope: Probability to drop the packet between the min and the max. The drop probability increases linearly (with a certain slope) with the queue size.

Note: The 3550 switch does not allow you to tune the min threshold, but only the max threshold. The min threshold is always hard set to 0. Any queue that is enabled for WRED on the 3550 always has a nonzero drop probability and always drops packets. This is the case because the min threshold is always 0. If you need to avoid packet drop at max, use weighted tail drop

threshold_1 and threshold_2 are two different max thresholds in order to provide two different services

Hope this help

Andrea