[c-nsp] BGP convergence with jumbo frames

Tony Li tony.li at tony.li
Mon Aug 2 03:01:07 EDT 2004

How are your huge processor buffers set up?

I would not expect a larger MTU/MSS to have much of an
effect, if at all.  BGP is typically not constrained by
throughput.  In fact, what you may be seeing is that
with really large MTUs and without a bigger TCP window,
you're turning TCP into a stop and wait protocol.


On Aug 1, 2004, at 10:42 PM, Pete Kruckenberg wrote:

> Spent some time recently trying to tune BGP to get
> convergence down as far as possible. Noticed some peculiar
> behavior.
> I'm running 12.0.28S on GSR12404 PRP-2.
> Measuring from when the BGP session first opens, the time to
> transmit the full (~128K routes) table from one router to
> another, across a jumbo-frame (9000-bytes) GigE link, using
> 4-port ISE line cards (the routers are about 20 miles apart
> over dark fiber).
> I noticed that the xmit time decreases from ~ 35 seconds
> with a 536-byte MSS to ~ 22 seconds with a 2500-byte MSS.
>> From there, stays about the same, until I get to 4000, when
> it beings increasing dramatically until at 8636 bytes it
> takes over 2 minutes.
> I had expected that larger frames would decrease the BGP
> converence time. Why would the convergence time increase
> (and so significantly) as the MSS increases?
> Is there some tuning tweak I'm missing here?
> Pete.
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