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

Frank Bulk frnkblk at iname.com
Fri Apr 24 12:47:34 EDT 2009


I know what you're feeling.  I had a case open with TAC to diagnose why were
getting input drops, and they wanted me to packet capture all the traffic
going to the interface to see if I could identify what traffic was
generating the microburst....except the volume is 40 to 60 Mbps and the
drops may not show up for an hour at a time.  How does one correlate it?
Run "sh controllers $interface | inc rx_no_descriptors|rx_resource_error"
and "sh int $interface | inc ignored" every second?  The TAC engineer made
it sound trivial to match things up, but it's not.  So instead the TAC
engineer did a little more research and in the end attributed it to
microbursts.  

I don't really understand how Cisco can build a product (I'm using the
NPE-G2) that can't take deal with microbursts, especially if the other
interface is a Cisco product!  The RX ring on an NPE-G2 is a 128 -- why
wasn't it designed with 1024?  Rodney mentions working with the BU, but it
appears in the end it doesn't matter that input drops due to microbursts are
a fact with this product.  The Cisco TAC engineer was apologetic, but he
wasn't going to lobby the PM. 

Frank

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Dale Shaw
Sent: Thursday, April 23, 2009 11:38 PM
To: cisco-nsp at puck.nether.net
Subject: [c-nsp] The dreaded microburst - definition and troubleshooting

Hi all,

Is there a universally agreed upon definition for a 'microburst'?

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?

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.

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".

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? :-)

cheers,
Dale
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