From: Brian McGahan (bmcgahan@internetworkexpert.com)
Date: Thu Nov 29 2007 - 14:32:00 ART
Hi Eric,
Inter sub-AS communication behaves like a combination of iBGP and
EBGP. It is like iBGP in the manner that next-hop values are not modified
from the confederation edge, even when passing between sub-ASes. It is like
EBGP in the manner that routes learned from peers in a different sub-AS are
not subject to synchronization, and they can be advertised to other true
iBGP neighbors inside the device's own sub-AS. So yes, technically you can
get around synchronization, full-mesh, and route-reflection issues by
running each device in it's own sub-AS. Whether this is the "best" solution
though... it ultimately depends on what the specific task is asking for. In
the lab exam look at all your possible solutions, then pick the simplest one
that doesn't violate any of the requirements.
HTH,
Brian McGahan, CCIE #8593 (R&S/SP/Security)
bmcgahan@internetworkexpert.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
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> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> Eric Phillips
> Sent: Thursday, November 29, 2007 10:31 AM
> To: Cisco certification
> Subject: Confusion re: BGP Synch + Confederations
>
> 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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