IBGP Question --- Router Reflector or iBGP Mesh

Thu Jan 13 08:02:12 UTC 2005

Agree; but do not forget that you can alwys add direct connections between
clients (if I am not forgotten something).
If 2 clients have direct link between them, it may be a good practice to add
direct iBGP connection.

It means that iBGP topology should reflect (more or less) network one. Else
you can have non-optimal (but still consistant and correct) routing.

----- Original Message ----- 
From: "David Barak" <thegameiam at yahoo.com>
To: <nanog at merit.edu>
Sent: Wednesday, January 12, 2005 4:20 AM
Subject: Re: IBGP Question --- Router Reflector or iBGP Mesh

>
>
> --- 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
> Need Geek Rock?  Try The Franchise:
> http://www.listentothefranchise.com
>
>
>
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