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

[Chrisy Luke <chrisy () flix net>]

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.

When you speak to a router that is directly attached and has BGP speaking
routers beyond it, it learns the networks of the interfaces attached to that
router. It then knows how to reach the networks directly behind it, connects
to the BGP speakers on those networks and so forth.

If you can't speak to a BGP router, you time out and try again later, when
perhaps you have learnt a route to it via other means.

With OSPF it builds up this information at startup from it's neighbours,
all of which keep a complete map of the network, all of which must be
learnt, the only difference is the mechanism is more propogatative than
discoverish. You can frig this of course by using BGP reflectors.

> How does this affect the hierarchy of the network since all iBGP speakers 
> must be fully meshed?

iBGP doesn't need to be fully meshed to work, even without reflection. The
results are perfectly predictable. A machine that only needs to know
internal routes doesn't need to speak to any transit routers, for instance.

And in any case, the results are no worse than in the "my network died
and is booting up slowly" situation when you're running any other IGP. It
takes just as long for everything to converge.

Chris.
-- 
== chris () easynet net, chrisy () flix net, chrisy () flirble org
== Systems Manager for Easynet, part of Easynet Group PLC.