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Old 06-17-2009, 01:23 AM
Chris Chris is offline
 
Join Date: Jun 2009
Posts: 74
Default Why I cannot ping the route although it is in the route table?

My topology is 3 routers:


The question is I canít ping 172.16.100.1 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.
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Old 06-17-2009, 05:03 AM
David.C David.C is offline
 
Join Date: May 2009
Posts: 118
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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 12.0.1.0/24 to RTRB. RTRB will receive this with a next-hop of 10.0.0.2.
B will pass that through to RTRC. It will also advertise 192.168.0.0/24 and 192.168.1.0/24, since these are connected networks. RTRC will see the next-hop routes for all 3 as 172.16.1.1.
So, RTRC's routing table contains:
12.0.1.0/24 gw 172.16.0.2

Going the other way:
RTRC will advertise 172.16.100.0/24 to RTRB. RTRB will see next-hop is 172.16.0.1.
RTRB will advertise that network. Next hop will be 10.0.0.2.
So, RTRA's routing table will contain:
172.16.100.0/24 gw 10.0.0.2

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

So, when you ping 172.16.100.1 from RTRA, the source IP is the one closest to the destination. In this case, source IP is 10.0.0.1...
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