From: Tarun Pahuja (pahujat@gmail.com)
Date: Thu Nov 29 2007 - 15:16:49 ART
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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