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@yahoo.com> To: <nanog@merit.edu> Sent: Wednesday, January 12, 2005 4:20 AM Subject: Re: IBGP Question --- Router Reflector or iBGP Mesh
--- 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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