IBGP Question --- Router Reflector or iBGP Mesh

David Barak thegameiam at yahoo.com
Wed Jan 12 12:20:14 UTC 2005

--- Alexei Roudnev <alex at relcom.net> wrote:

> Are you sure? RR should just distribute routes.
> RR do not make any route decisions, and (btw) iBGP
> do not make route
> decisions - they are mostly based on IGP routing.
> All iBGP + RR are doing
> is:
> - tie external routes to internal IP;
> - distribute this information using iBGP mesh, RR's
> etc.
> - receive this information and set up routing using
> internal IP (which are
> routed by IGP protocls).
> End routers receives iBGP routes and uses IGP (OSPF
> or EIGRP or anything you
> use) for route decisions (of course, we can image
> exceptions, but normally ,
> it works so that all decisions are based on IGP
> routing). Most important
> decisions are done , where routes are emitted from
> EBGP into iBGP, others -
> by iGP; which decisions are done by RR's themself?

The primary decision made by a route-reflector is the
same decision which would be made by multiple routers
in an iBGP full-mesh: which exit point should this
router use to reach a specific netblock.

Leaving aside for the moment any manipulation of
multipath, each router will run the BGP route
selection algorithm on each route learned.  If
multiple routes are learned to a given destination,
only one will be inserted into the RIB.  The standard
behavior for a router is to only pass on those routes
which have been accepted into the RIB.

So if you have this network

C1 -R1--R2-C2
     |   |
C1 -R3--R4-C3

And R1 is the only route-reflector (yeah, yeah, bad
design - it's just an example), R4 will only learn
about the path to C1 through R1, and might route
traffic along the R4->R2->R1->C1 path rather than
along the R4->R3->C1 path which would be preferred by
an iBGP full-mesh.

The upshot of this is the following (drumroll):
route reflectors are a wonderful thing, but make sure
that their topology reflects and respects your
underlying IP network topology.  If you don't, you can
get unpleasant consequences.

David Barak
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