> >Hey Pierre, > >In this case, AS103 breaks the loop. Keep in mind that in BGP you are >only allowed to advertise your best path for a particular NLRI. In the >case of R3, it learns 172.16 via EBGP from 102, and IBGP from R4. It >prefers the EBGP route due to E vs I preference and only advertises that >outbound which as 102 ignores to due path loop. (in cisco's case, a semi >proprietary split horizon behavior likely stifles the re advertisement but >that point is immaterial wrt this issue). R4 does the same thing, >preferring the AS100 advertisement. Hence, R2 never sees the 172.16 from >R0 and vice versa. When you initially trigger updates, timing issues may >allow a route to flow through such that R3 likes R4's 172.16 prior to >learning R2's 172.16 which results in it initially advertising the route, >and subsequently withdrawing it. > >Hope that helps. > >Pete > > > >At 10:26 PM 8/10/2002 +0000, you wrote: >>This is the network: >> >>AS101(R1) --- AS102 (R2) --- AS103 (R3)--- (R4)-----AS100(R0)---- to AS101 >>(R1) >> >>(R3 and R4 are IBGP neighbors) >> >>I have route 172.16.0.0 on AS101 (R1) that is advertised via BGP. >>When i do a "show ip bgp", the only routers that have two paths to >>172.16.0.0 are R3 and R4. R0 and and R2 only have on path >> >>Given that I have a loop, I would have expected R0 to also have a path to >>172.16.0.0 via As103 and R2 to also have a path to 172.16.0.0 via AS103. >> >>Do I have a configuration problem or is this the default behavior of BGP >>that it won't accept another path if it already has the information from a >>directly connected neighbor? Is this to prevent loops?
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