Tuesday, June 1, 2010
Understanding mls qos srr-queue input threshold command
Can someone explain the meaning of the two parameters threshold-percentage1 and threshold-percentage2 of the command mls qos srr-queue input threshold? What is the effect, an when is it recommended to modify this?
This example shows how to configure the tail-drop thresholds for the two queues. The queue 1 thresholds are 50 percent and 100 percent, and the queue 2 thresholds are 70 percent and 100 percent:
Switch(config)# mls qos srr-queue input threshold 1 50 100
Switch(config)# mls qos srr-queue input threshold 2 70 100
QoS uses the CoS-to-threshold map or the DSCP-to-threshold map to decide which class of service (CoS) or Differentiated Services Code Points (DSCPs) values are mapped to threshold 1 and to threshold 2. If threshold 1 is exceeded, packets with CoS or DSCPs assigned to this threshold are dropped until the threshold is no longer exceeded. However, packets assigned to threshold 2 continue to be queued and sent as long as the second threshold is not exceeded.
Each queue has two configurable (explicit) drop threshold and one preset (implicit) drop threshold (full).
You configure the CoS-to-threshold map by using the mls qos srr-queue input cos-map global configuration command. You configure the DSCP-to-threshold map by using the mls qos srr-queue input dscp-map global configuration command.
If thresold 1 in crossed over 50 percent, it starts dropping packets known as SPD to avoid congestion ( e.g. it drops 1 out of 10). If the input still grows say to 80 it drops packet ( e.g drops 6 out of ten) more thans what it was dropping when it crossed 50. When it reaches 100 it drops every packet.
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