AT&T NYC

Tue Sep 3 07:55:03 UTC 2002

On Mon, Sep 02, 2002 at 11:16:51PM -0400, alex at yuriev.com wrote:
> > 
> > To carry the bgp next-hops around the network? You could add in statics
> > for every next-hop on every router, but this kind of configuration is
> > complex and prone to errors such as loops in relatively minor cases.
> > statically routing every next-hop "does not scale". Not to mention
> > that many of us like the "compare igp metric" portion of the BGP decision
> > tree.
> 
> Rubbish again.
> 
> *Every* interface that you bring up has a connected route. You redistribute
> those routes into IGP. You redistgribute statics from that router into IGP.
> Nail those routes into bgp and set internal community on it. 
> 
> network xxx.yyy.zzz.www mask ppp.hhh.ooo.lll route-map set-igp-community.

So how does this provide equivalent functionality to "compare igp
metric"? I think there are a lot of folks out there who might like
to do the whole nearest-exit thing. Even if you went to the trouble
of setting up route-maps to your heart's content and managed to get
each router to prefer paths from the nearest exit router, it wouldn't
do you much good when a link failure turns that "nearest" into
"furthest" but the iBGP session stays up.

I think maybe the word "need" is being taken a little too seriously
here. No, you don't NEED a separate IGP to make BGP work. But then
again, you don't NEED a lot of things to make a network go in its
most basic form. However, without some of those "unnecessary" things
you might not find it to perform quite to your liking either. For my
network, I'd much rather deal with some extra SPF calculations than
slow convergence and playing route map games to get things like
nearest-exit working.

Links and loopbacks => IGP
Everything else => BGP

But then, nobody ever accused any two engineers of having the same
personal preferences...

-c