nanog mailing list archives
Re: IBGP Question --- Router Reflector or iBGP Mesh
From: David Barak <thegameiam () yahoo com>
Date: Wed, 12 Jan 2005 04:20:14 -0800 (PST)
--- 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 __________________________________ Do you Yahoo!? Yahoo! Mail - Helps protect you from nasty viruses. http://promotions.yahoo.com/new_mail
Current thread:
- RE: IBGP Question --- Router Reflector or iBGP Mesh, (continued)
- RE: IBGP Question --- Router Reflector or iBGP Mesh Erik Haagsman (Jan 07)
- RE: IBGP Question --- Router Reflector or iBGP Mesh Eric Kagan (Jan 10)
- RE: IBGP Question --- Router Reflector or iBGP Mesh Erik Haagsman (Jan 11)
- Re: IBGP Question --- Router Reflector or iBGP Mesh Philip Smith (Jan 11)
- Re: IBGP Question --- Router Reflector or iBGP Mesh Daniel Roesen (Jan 11)
- Re: IBGP Question --- Router Reflector or iBGP Mesh Erik Haagsman (Jan 11)
- Re: IBGP Question --- Router Reflector or iBGP Mesh Alexei Roudnev (Jan 12)
- Re: IBGP Question --- Router Reflector or iBGP Mesh Iljitsch van Beijnum (Jan 12)
- Re: IBGP Question --- Router Reflector or iBGP Mesh Erik Haagsman (Jan 12)
- Re: IBGP Question --- Router Reflector or iBGP Mesh Alexei Roudnev (Jan 12)
- Re: IBGP Question --- Router Reflector or iBGP Mesh David Barak (Jan 12)
- Re: IBGP Question --- Router Reflector or iBGP Mesh Alexei Roudnev (Jan 13)
- RE: IBGP Question --- Router Reflector or iBGP Mesh Erik Haagsman (Jan 07)
- Re: IBGP Question --- Router Reflector or iBGP Mesh Iljitsch van Beijnum (Jan 11)