[j-nsp] MPLS-in-MPLS mtu

[Jeff S Wheeler]

On Mon, 2007-04-16 at 10:32 +0100, Alex wrote:
> Jared,
> Max BGP message size is 4096 bytes anyway and I cannot possibly see how 9K 
> MTU can further increase tcp performance in case of iBGP in comparison with 
> MTU of 4470.
> Rgds
> Alex

It's sometimes foolish to run iBGP sessions with a TCP MSS > 536 bytes.
This is the minimum allowed for IP networks, and there should never be a
router transporting your iBGP session which cannot forward TCP segments
conforming to this minimum MSS value.

A topology change might happen and cause your iBGP session, which under
nominal conditions is transported over a 4470 MTU path, to pass across a
1500 MTU link.  Now your TCP stack is sending packets larger than the
path MTU, which will either be fragmented or discarded.

The minimum values are the safest ones for iBGP, in my opinion; and the
benefit to using any larger TCP MSS for iBGP is poor given the CPU time
your router spends in its TCP/IP stack vs other tasks like calculating
routes.  You could argue that converge time is improved if the iBGP TCP
session is more efficient, but that benefit goes away the first time
your operators page you at 4am to find out why an iBGP session flaps
when a topology change favoring a lower-MTU core link happens.  :-)

-- 
Jeff S Wheeler <jsw at inconcepts.biz> +1-212-981-0607
Sr Network Operator  /  Innovative Network Concepts