[c-nsp] Output drops on 2960

John Gaffney jgaffney at nan.com
Fri Feb 5 10:16:49 EST 2016


Thanks for the feedback. That was actually what I was starting to think was to shut off mls qos. This switch has been in place so long nobody knows why it is the way it is. I do need to trust dscp tags that run though it, do you or anybody else know if I shut of mls qos if I can still trust dscp/not strip tags traversing the switch?

Looking more in depth at the mls q's I can see only 2 of the q's are really dropping traffic. The others are fine. 

  output queues dropped:
 queue:    threshold1   threshold2   threshold3
 queue 0:           0           0           0
 queue 1:       68722           0           0
 queue 2:      163969           0           0
 queue 3:           1           0           0



-----Original Message-----
From: Saku Ytti [mailto:saku at ytti.fi] 
Sent: Friday, February 5, 2016 9:17 AM
To: John Gaffney <jgaffney at nan.com>
Cc: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] Output drops on 2960

Hey John,

> Switch is a straight layer 2, no layer 3 functionality short of mls qos and some dscp mutation. Anybody seen this kind of thing before? Any commands that may help point me towards why these drops are happening? Anybody ever fatten the q's on these things?

Turning on 'mls qos' without through understanding of the platform (which is hard to acquire, as docs are sparse), almost invariably makes things progressively worse.
It'll split the already tiny buffers into smaller buffers, and you likely have significant amount of buffer memory now carved which you cannot use.

Read https://supportforums.cisco.com/document/31581/egress-qos#_Toc191205672
and redesign your QoS config, and try avoid having many queues, if you can live with one great, two is doable but anything else, can you really  absolutely justify it?


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