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RE: BGP Path Question posted 08/25/2007
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Only the best path (shown by the ">") is advertised  to the neighbor.
R1 wont advertise the path back to  its neighbor R3, since it is   using the
advertisement from R3 (that same neighbor)  in the first place == Split
Horizon
Joel
CCIE#18128



> From: CiscoKid@xxxxxxxxxxxxxxx> To: ccielab@xxxxxxxxxxxxxx> Subject: BGP
Path Question> Date: Sat, 25 Aug 2007 21:29:01 -0300> > R2 (AS2000)>
----------fa0/0----------> | |> |e0 |fa0/0> R8 (AS8000) R1(AS1000)> |e1
|fa0/1> | |> ----------fa0/0----------> R3 (AS3000)> > eBGP Peerings:> R8--R2>
R8--R3> R1--R2> R1--R3> > Situation:> R8 is advertising 80.0.0.0/8 with all
default attributes.> > Result:> > R2-2610XM#show ip bgp> Network Next Hop
Metric LocPrf Weight Path> * 80.0.0.0 10.10.10.1 0 1000 3000 8000> i> *>
10.10.10.8 0 0 8000 i> > > > R1-2611XM#show ip bgp> Network Next Hop Metric
LocPrf Weight Path> * 80.0.0.0 10.10.10.8 0 2000 8000 i> *> 20.20.20.8 0 3000
8000 i> R1-2611XM#> > > R3-2610XM#show ip bgp> Network Next Hop Metric LocPrf
Weight Path> *> 80.0.0.0 20.20.20.8 0 0 8000 i> > > > Question: > > Why is R3
not seeing both paths? Is R1 not advertising the longer > path to R3 because
it is aware that R3 already has a shorter path?> > Even if that was the case I
would have assumed both paths would be > seen by R3 but the shortest path via
8000 would be selected as best.> > > When I debug "ip bgp updates" R1 does not
send an update to R3 for > that path unless, R3 sends a withdrawal of its
existing path.> >
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