[77043] in North American Network Operators' Group
Re: IBGP Question --- Router Reflector or iBGP Mesh
daemon@ATHENA.MIT.EDU (David Barak)
Wed Jan 12 07:20:43 2005
Date: Wed, 12 Jan 2005 04:20:14 -0800 (PST)
From: David Barak <thegameiam@yahoo.com>
To: nanog@merit.edu
In-Reply-To: <0e0901c4f87d$99768ba0$6401a8c0@alexh>
Errors-To: owner-nanog-outgoing@merit.edu
--- Alexei Roudnev <alex@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
Need Geek Rock? Try The Franchise:
http://www.listentothefranchise.com
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