[c-nsp] BGP convergence with jumbo frames

[Pete Kruckenberg]

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.