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North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: IBGP Question --- Router Reflector or iBGP Mesh
--- Alexei Roudnev <[email protected]> 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
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