I believe that the problem for this dilemma is fairly simple - the use of path-mtu-discovery allows you to use the maximum mtu size supported on the network.
Amos On Apr 16, 2007, at 3:21 PM, Jared Mauch wrote: > On Mon, Apr 16, 2007 at 06:26:38AM -0400, Jeff S Wheeler wrote: >> 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. :-) > > If you don't have sufficent control over your network > to know the underlying mtu of the links (be they l2vpn-type or > otherwise carrier provided) you have some serious issues. > > There was a presentation years ago at IETF where Cisco showed > the performance increases of enabling path mtu discovery and how this > took those bgp message sizes grew from the cisco default to match your > link mtu. > > I'm advocating a consistent internal MTU for your network, be that > 1500, 4470, or something larger. If the underlying transport does not > support it and you are dealing with broken host stacks from your > vendors > then you should discontinue using their equipment until they > repair these critical defects. > > Scaling your bgp update messages to something larger than 500 bytes > can have a significant win in route convergence as we're all carrying > voip and other similar sensitive traffic on our networks (even if > we don't > know what all that sensitive traffic is). > > - jared > > -- > Jared Mauch | pgp key available via finger from [EMAIL PROTECTED] > clue++; | http://puck.nether.net/~jared/ My statements are > only mine. > _______________________________________________ > juniper-nsp mailing list [email protected] > https://puck.nether.net/mailman/listinfo/juniper-nsp _______________________________________________ juniper-nsp mailing list [email protected] https://puck.nether.net/mailman/listinfo/juniper-nsp
