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

Виктор Моисеев vim at psu.ru
Thu Apr 7 01:56:54 EDT 2016

If you see Output drops it means interface is Congested. That's the purpose
of counter.
And tradeoff of setting high thresholds by default is that you will get
noisy neighbors problem.
Any port with excessive traffic can eat up whole common pool, so other ports
will start drop legitimate traffic.

Imho that's why you should increase buffer based on traffic requirements
only to particular ports.

You can try some heavy linerate UDP traffic from 10G to 1G interface and
you'll fill any buffer.
And you'll see Output drops increment on 9G speed )

How much buffer space is common for all ports or reserved for particular
port is dependent on platform/config.
What size exactly in packets/bytes that buffer is - different question. 

Best regards,
Victor Moiseev

> -----Original Message-----
> From: cisco-nsp [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of
> Victor Sudakov
> Sent: Thursday, April 07, 2016 9:26 AM
> To: Saku Ytti
> Cc: cisco-nsp at puck.nether.net
> Subject: Re: [c-nsp] Output Drops Due to QoS and threshold size
> Saku Ytti wrote:
> > On 6 April 2016 at 20:09, Victor Sudakov <vas at mpeks.tomsk.su> wrote:
> >
> > > Why is it *not* the default configuration?
> >
> > I don't know why the defaults are so broken, small cats are worse
> with
> > mls qos enabled than without. And the common wisdom is if you have
> > microbursts MLS QoS will only make it worse, while counter-
> intuitively
> > even with single queue with mls-qos you get more buffers than without
> > it.
> Still I don't understand the idea of configuring a threshold >100%.
> If for example IOS assigns 200 buffers to a port which are equally
> distributed between 4 queues ("buffers 25 25 25 25"), whis means each
> queue gets 50 buffers. If Thres1 in Q1 is 100%, it means 50 buffers.
> If Thres1 in Q1 is set to 1000%, this would mean 500 buffers. Where
> are the 500-200=300 additional buffers borrowed? From the common pool?
> --
> Victor Sudakov,  VAS4-RIPE, VAS47-RIPN
> sip:sudakov at sibptus.tomsk.ru
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