eBGP, iBGP, injecting networks

Fri Feb 20 23:51:15 UTC 2004

Well, you sort of can with confederations (internally) but the external 
view is still the single advertised ASN.

At 07:10 PM 2/20/2004, william(at)elan.net wrote:

>Note - I got confused by the subject and everything myself. The routes you
>have locally would not be from IBGP but just directly through IGP (i.e.
>OSPF or EIGRP etc). I don't think you can really do IBGP if routers are
>not configured with the same ASN.
>
>On Fri, 20 Feb 2004, william(at)elan.net wrote:
>
> >
> > Ok. The way I read this is that you're redundant as far as one of your
> > upstream links going down - it'd not cause complete meltdown as that
> > router that had that link would still be announcing that space to the
> > other router (over EBGP) and then to the net.
> >
> > What you're worrying then is what happens if actual router is down, right?
> > But that begs the question of how you're getting the routes that router is
> > announcing in the first place. Is it coming from some other "edge" router
> > (that is also talking over local net to your 2nd core router)?
> >
> > If so each of your routers has complete local routes table through IBGP
> > and you are not announcing it all because you're using static "network"
> > statements in BGP config. In that case my suggestion would be to drop EBGP
> > connection between routers and have each router announce entire ip space
> > but put up 'as-path prepend' statements with the other adding the other
> > router's ASN for routes that you want to be considered as being primary
> > from that other router. Now exact configuration suggestion would depend on
> > what hardware the routers are, i.e. is it cisco, etc.
> >
> > P.S. I've never been in situation of having to merge two ASN's or in 
> situation
> > you describe, so possibly people who have would have better suggestions.
> >
> > On Fri, 20 Feb 2004 isaac at ravengate.net wrote:
> >
> > >
> > > greetings list,
> > >
> > > hoping someone can hook me up on the right way to do this.
> > >
> > > ---
> > >
> > > we have two ASN's we control.
> > >
> > > we have two border/edge routers (1 in each ASN) that talks to a
> > > different backbone provider.
> > >
> > > the two border routers peer with eachother over eBGP and also are in
> > > the same OSPF process.  (we are working to merge them into the same
> > > BGP ASN)
> > >
> > > my question is this:
> > >
> > > how do we achieve router redundancy between these two routers?
> > >
> > > currently if we lose a transit link, the traffic will flow fine out
> > > the other pipe.
> > >
> > > but we don't have BGP network statements in router 2 that exist in
> > > router 1 and we don't have BGP network statements in router 1 that
> > > exist in router 2.
> > >
> > > so the routes injected into BGP from router 1 will get withdrawn right
> > > if router 1 dies?
> > >
> > > is it a problem to announce the same networks from two different eBGP
> > > peers to two different upstreams?
> > >
> > > ------
> > >
> > > if you are still reading, thanks!
> > >
> > > to clearify some more-
> > >
> > > current setup:
> > >
> > > current setup:
> > >
> > > ASN 1 (we're not Genu!ty- just using for an example)
> > >
> > > :)
> > >
> > > ASN 1 injects all of its own space and announces this space to
> > > Above.net and ASN 2
> > >
> > > ASN 2 injects all of its own space and announces this space to Savvis
> > > and ASN 1.
> > >
> > > so stuff out on the net looks like:
> > >
> > > 1 6461 etc etc
> > >
> > > and
> > >
> > > 1 2 6347
> > >
> > > -------
> > >
> > > 2 6347 etc etc
> > >
> > > and
> > >
> > > 2 1 6461 etc etc
> > >
> > > -------
> > >
> > > so, you see we are prepending on of our AS's on the way out.
> > >
> > > the problem is tho, we only have 1 router in each respective Autonmous
> > > System injecting address space.  if we lose that router, we lose
> > > announcing that ASN's space.
> > >
> > > is it totally going to cause probs to have routes originating from two
> > > different AS's?  routing loops would be a real drag.
> > >
> > > what about having an iBGP router in AS 1 inject the same space as the
> > > border router in AS 1?  this other router also peers with AS 2....
> > >
> > > thanks a lot!
> > > jg

Vinny Abello
Network Engineer
Server Management
vinny at tellurian.com
(973)300-9211 x 125
(973)940-6125 (Direct)
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