BGP as an IGP (Was Re: IGPs in use)

Chrisy Luke wrote:

Paul G. Donner wrote (on Oct 13):
> If BGP rides on TCP, how are the TCP sessions built if BGP itself is used
> as the IGP?

Same way as it does when you take next-hops from any other IGP. The fact
that one already has a route to ones' directly attached networks.

In other words, the interior routing protocol is either static
or a combination of statically configured routes and the likes
of ARP/ES-IS/other neighbour discovery protocols.

The key point is that, in a router talking iBGP, the route to the
NEXT_HOP received by an iBGP neighbour *MUST* be known through
means other than BGP. This is not to say that the route need
be dynamic -- a static default route would do just fine.

  Sean.

Sean M. Doran wrote (on Oct 14):

The key point is that, in a router talking iBGP, the route to the
NEXT_HOP received by an iBGP neighbour *MUST* be known through
means other than BGP. This is not to say that the route need
be dynamic -- a static default route would do just fine.

Not necessarily. You just get a more pronounced stepping effect when
you learn routes whose next-hops are in the same protocol.

I don't recommend it in a large network though. It gets very messy.

Chris.

Either you have to configure next-hop-self or use static or use other IGP,
otherwise you'll end up with flapping routes in your network. BGP cannot
use a route for next-hop-self address derived by itself, as far as I know.

/Hakan

Hakan Hansson wrote (on Oct 14):

Either you have to configure next-hop-self or use static or use other IGP,
otherwise you'll end up with flapping routes in your network. BGP cannot
use a route for next-hop-self address derived by itself, as far as I know.

Well it does and can. But then I suppose it depends on the arbitrary
restrictions certain vendors place on you.

Chris.