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Chris 06-17-2009 01:23 AM

Why I cannot ping the route although it is in the route table?
My topology is 3 routers:

The question is I canít ping from RouterA although the route exists in RouterAís route table. The packet gets dropped at Router B. Maybe something to do with source address etc. I'm currently doing lab after lab on bgp so hopefully i'll nail it soon.
Sorry if the answer is obvious but it isn't so obvious to me at the moment.

PS: if you read my post in hardware, i'm using a 2509 access server for router B.

David.C 06-17-2009 05:03 AM

Router B is a transit AS. This is something to avoid when you're a dual-homed subscriber, but something that's very important for the Internet to work.

It's receiving the route as advertised by Router A, and sending it along to Router C. Remember: "A BGP router will not advertise routes learned via IBGP to other IBGP peers". The opposite holds true: "A router will advertise routes learned via EBGP to both IBGP and EBGP peers."

Your problem is indeed a return route:

A advertises to RTRB. RTRB will receive this with a next-hop of
B will pass that through to RTRC. It will also advertise and, since these are connected networks. RTRC will see the next-hop routes for all 3 as
So, RTRC's routing table contains: gw

Going the other way:
RTRC will advertise to RTRB. RTRB will see next-hop is
RTRB will advertise that network. Next hop will be
So, RTRA's routing table will contain: gw

(routing tables are just the BGP learned routes, btw, since connecteds and statics don't come into play)

So, when you ping from RTRA, the source IP is the one closest to the destination. In this case, source IP is

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