[c-nsp] Source address on BGP peering set up

[Rick Cossey]

On Fri, 14 Jan 2005 15:45:30 +0200, Mark Tinka
<mtinka at africaonline.co.sz> wrote:
> On Friday 14 January 2005 15:14, Piltrafilla wrote:
> > Hi people,
> 
> Hello.
> 
> >
> > Anyone knows how BGP on a Cisco router choose source IP address for
> > peering establishment if no "update-source" command is applied to
> > neighbor? Is it only the primary IP address on the closest interface
> > to neighbor?
Debug is your friend if you can't find the info otherwise. It will
show you the source of the packet.
> 
> Without manually specifying your source interface, BGP will find and use the
> Loopback interface with the highest IP address. Failing that, it will take
> the next highest IP address configured on the router (regardless of
> interface).
This is not true, BGP will always "source" from the outgoing interface
unless you specify an interface with update-source. The source of the
packets is much different than the Router-ID which is what I think you
are confusing here. Router-ID is really only used as BGP best
route(path) selection in a tie-breaker scenario when everything else
is equal. The source address is used to establish peering(create the
TCP session)

> 
> This can become a problem, especially if you keep adding/removing IP addresses
> on your router (customers, new routers, new segments, e.t.c.), hence the
> elegance of 'update-source'.
> 
> Mark.
> 
> _______________________________________________
> cisco-nsp mailing list  cisco-nsp at puck.nether.net
> https://puck.nether.net/mailman/listinfo/cisco-nsp
> archive at http://puck.nether.net/pipermail/cisco-nsp/
>