From: Eric Phillips (ephillips@squick.cc)
Date: Thu Nov 29 2007 - 21:33:51 ART
Thanks Brian, Bob, and Taurn. That explains it perfectly. I knew there was
some sort of EBGP/IBGP duality going on there, but that seems to defeat the
purpose of synchronization because R3 might not know how to get to the next
hop because the next-hop is not modified. I am very glad I labbed that up
though, I would have never understood that without.
Again, thanks for the help. I try to do my best researching issues before
posting here, because I respect the time you all put into helping us out.
Thanks,
Eric
On 11/29/07, Tarun Pahuja <pahujat@gmail.com> wrote:
>
> Eric,
> The previous post must have answered your question. Additionally,
> In Confederations EBGP preserves LOCAL_PREF, MED and NEXT_HOP.
>
> You can read more:
>
> http://www.cisco.com/warp/public/459/bgpfaq_5816.shtml#twelve
>
> HTH,
> Tarun
>
>
>
> On Nov 29, 2007 11:31 AM, Eric Phillips <ephillips@squick.cc> wrote:
> > Hey all,
> >
> > I am seeing a really strange outcome when I combine BGP synchronization
> and
> > confederations. I have not been able to find an answer to what I am
> seeing
> > because every book I refer to just says to turn synchronization off.
> >
> > Assume I have four routers configured in a row:
> > R1 -- R2 -- R3 -- R4
> >
> > R1 is in AS 100 and has a lot of routes it is advertising.
> > R2-R4 are in AS 200, which is also a confederation.
> > R2 is in AS 65502
> > R3 and 4 are in AS 65501.
> >
> > Each router peers only with its neighbor.
> >
> > AS 200 also has synchronization enabled in it.
> >
> > As expected, nearly all routes on R4 are in the BGP table, but not
> selected
> > as best, and not put in the routing table.
> >
> > The strange behavior I am seeing is R3 has all of the routes coming from
> R1
> > and R2 marked as best and in the routing table, but with an AD of 200
> > (IBGP). But as expected, the advertised loopbacks from R4 are not
> marked
> > best because they were learned via IBGP. So it appears synchronization
> is
> > treating them like EBGP routes, but they have the AD of IBGP routes.
> >
> > So my question is, does BGP synchronization think that since the routes
> were
> > learned from another AS within the confederation that they are EBGP
> routes
> > and install them even though no other routing protocol has advertised
> them?
> > If so, that would make it possible to basically circumvent
> synchronization
> > if you made every router it's own sub-AS within a confederation,
> right? And
> > as a slight formality, the link between R2 and R3 is considered EBGP,
> > right? So if a question said "do not use EBGP" that would rule
> > confederations out; right?
> >
> > Thanks,
> >
> > Eric
> >
> > _______________________________________________________________________
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