[c-nsp] Many "Total output drops", no congestion

[Victor Sudakov]

Dear Colleagues,

There is a huge number of "Total output drops" on one of the
interfaces on a C3560X-24P with mls qos enabled. The egress traffic is
not more than 40-50 Mbits/s according to mrtg (the interface being in
Full-duplex, 100Mb/s mode). Only the most loaded of the 4 egress
queues was dropping packets.

I have been able to eliminate the output drops by following the
recommendations in
http://www.cisco.com/c/en/us/support/docs/switches/catalyst-3750-series-switches/116089-technote-switches-output-drops-qos-00.html
and namely by configuring

mls qos queue-set output 1 threshold 2 3100 3100 100 3200

My question is: how is it possible that there were so many drops
(hundreds per second) while the interface was so far from being
congested. My understanding so far has been that there can be no drops
before the congestion is reached. The interface configuration is
simple:

interface GigabitEthernet0/2
 description V-Node S Port 4 BPTOiK
 switchport trunk encapsulation isl
 switchport mode trunk
 power inline never
 srr-queue bandwidth shape 0 0 0 0
 mls qos trust cos
!

Thank you very much in advance for shedding light on this.

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