AT&T NYC

Mon Sep 2 19:56:11 UTC 2002

> > Link-state protocols are evil, and when they break, they *really* break.
> > I still do not see a compeling argument for not using BGP as your IGP.
> > 
> > Alex
> 
> iBGP is only one half of an IGP. It is the "where to go" half.
> You still need some other igp (isis, ospf, rip, static routes, etc) for
> the "how to get there" half.
> 
> Most large providers carry next-hops (loopbacks, border /30 (or /31), etc)
> around in either isis or ospf, and carry the remainder in iBGP.
> 
> The reason?
> 
> With link-state, one interface flap can mean doing SPF on every route.
> If "every route" is only a couple hundred, rather than 100K, you fare
> better when circuits are flapping. At that point, comparing the precomputed
> metric amongst 100k routes is (imho) rather trivial, especially when
> "igp metric" is a few steps down the decision tree.
> 
> In all practicality, you need to haul, at least, eBGP routes around in iBGP,
> you need some kind of other igp to jumpstart iBGP, and is advised that this
> other igp have some concept of metric or cost to be able to give BGP
> some hints. Whether you choose to make your non-BGP igp lean and mean
> (disabling synchronization, with the attendant caveats) or fat and happy
> (and suffer the consequences during repeated link state changes), is up
> to the reader, but you still really need two igps.

Rubbish.

Every interface that you have has a connected route or static route. Those
routes are distributed into your favourite BROKEN[1] IGP. There is no need
to do that. Originate that route as a BGP route with set-igp-community
route-map on itm if you want to do something only with routes that are
internal to your network. Your customers wont care about convergence of BGP
if your network does not break due to the funky IGP implementations.

Alex

[1] Any protocol that allows a non-affected path to be taken out of service
by an affected path is BROKEN BY DESIGN. Any network that requires a
protocol, which is broken by deisgn to function, is a badly designed
network.