[c-nsp] The dreaded microburst - definition and troubleshooting

[Rodney Dunn]

On Fri, Apr 24, 2009 at 02:38:01PM +1000, Dale Shaw wrote:
> Hi all,
> 
> Is there a universally agreed upon definition for a 'microburst'?

None that I have ever seen because it's all relative.

> 
> Is there a defined time measurement - i.e. 5ms, 10ms, 50ms, 100ms,
> 1000ms - during which a certain bps or pps threshold must be
> met/exceeded?

No because it's dependent on the features and traffic profiles
through the entire box.


> 
> Does anyone have any tips for troubleshooting microbursts,
> particularly in relation to the c7200 platform exhibiting "no buff"
> drops? We're going to capture some data (w/SPAN on an adjacent switch)
> but it would be nice to be able to look at the data and somehow marry
> it up with incrementing drop counters on the affected c7200 interface.

#1 issue with this box as high speed switches have been connected to them.

What does 'show c7200' say?



> 
> It would be nice to be able to explain such drops like "within the
> measurement window, we saw traffic at bps/pps rate x, and we know that
> anything beyond bps/pps rate y will result in drops".

The best you could get is that the rx ring, IIRC from 'show controller',
is 128. If you overrun that you will see an overrun/drop. There are a lot
of internal complexities to how interrupts are handled for high speed
switching in software. It's not perfect. There are times when interrupts
need to be disabled and any short time is enough to overrun the rx ring.
I worked with the BU to try and increase that rx ring depth but it was
much more complex than it appears to make that happen.

s> I suppose it's platform-specific, but how does one come up with an
> accurate benchmark? Is such precision just wishful thinking in the
> murky world of microbursts? :-)

Honestly, wishful thinking. It varies by plaform, feature configuration,
user interaction, etc.

Rodney


> 
> cheers,
> Dale
> _______________________________________________
> cisco-nsp mailing list  cisco-nsp at puck.nether.net
> https://puck.nether.net/mailman/listinfo/cisco-nsp
> archive at http://puck.nether.net/pipermail/cisco-nsp/