[c-nsp] Output Drops Due to QoS and threshold size

Victor Sudakov vas at mpeks.tomsk.su
Sat Apr 2 06:30:40 EDT 2016

Dear Colleagues,

I am now struggling with output drops on a 3560X. There is a good
document at http://www.cisco.com/c/en/us/support/docs/switches/catalyst-3750-series-switches/116089-technote-switches-output-drops-qos-00.html
The interface is not congested so it recommends increasing the buffer
and threshold values for the queue that drops packets:

	In this example, queue-set 2 and queue 1 thresholds are changed. Both
	threshold 1 and threshold 2 are mapped to 3100 so that they can pull
	buffer from the reserved pool if required.

	Switch(config)#mls qos queue-set output 2 threshold 1 3100 3100 100 3200

My question is, what's the tradeoff? Why is the default threshold size
the mere 100 and not 3100? What will happen if I make it 3100 on all
queues in all the switches and forget about drops forever? Is it

Thanks a lot for any input.

Victor Sudakov,  VAS4-RIPE, VAS47-RIPN
sip:sudakov at sibptus.tomsk.ru

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