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Understanding CoS Tail Drop Profiles

Tail drop profile is a congestion management mechanism that allows switch to drop arriving packets when queue buffers become full or begin to overflow.

Tail drop profiles define the meanings of the loss priorities. When you configure tail drop profiles you are essentially setting the value for queue fullness. The queue fullness represents a percentage of the memory used to store packets in relation to the total amount that has been allocated for that specific queue.

The queue fullness defines the delay-buffer bandwidth, which provides packet buffer space to absorb burst traffic up to the specified duration of delay. Once the specified delay buffer becomes full, packets with 100 percent drop probability are dropped from the tail of the buffer.

You specify drop probabilities in the drop profile section of the CoS configuration hierarchy and reference them in each scheduler configuration.

By default, if you do not configure any drop profile, tail drop profile is in effect and functions as the primary mechanism for managing congestion. In the default tail drop profile, when the fill level is 0 percent, the drop probability is 0 percent. When the fill level is 100 percent, the drop probability is 100 percent.


The default drop profile associated with the packets whose loss priority is low cannot be modified. You can configure custom drop profile only for those packets whose loss priority is high.