[c-nsp] BGP MSS=576 bytes

[Seth Mattinen]

Dale Shaw wrote:
> Hi,
> 
> 2009/2/12 Antonio Soares <amsoares at netcabo.pt>:
>> This seems buggy to me :(
> 
> Bad documentation, at least. The IOS 12.4 *mainline* command reference
> and config guide waffle on and on about 'bgp transport' yet the
> command (BGP router config mode or per-neighbour) doesn't even exist
> in that train.
> 
> I suspect when it does exist (allegedly in 12.2(33)SRA, 12.2(31)SB,
> and 12.2(33)SXH and 12.4(20)T), the behaviour is as described -- BGP
> uses the BGP-specific TCP MSS settings. Without it, it uses the global
> default (PMTUD off), and you need the 'ip tcp path-mtu-discovery'
> global command to make it work.
> 
> I've tested it on IOS 12.4 mainline and even for directly connected
> neighbours, MSS is 516 bytes. Once 'ip tcp path-mtu-discovery' is
> switched on (on both peers), MSS is 1440 bytes.
> 

Just for fun I looked at my BGP mesh and found the following:

* Directly connected neighbors MTU=516
* Transit to upstreams MTU=1440 or 1460

This is under 12.4(16) and I've never touched 'ip tcp 
path-mtu-discovery' or per-neighbor MTU settings.

~Seth