Home NetBrain Product One-Skill-at-a-Time Search Mark Forums Read
Go BackThe Net-BrainerOne-Skill-at-a-Time Why I cannot ping the route although it is in the route table?

Thread Tools Search this Thread Display Modes
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 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.
Reply With Quote
Old 06-17-2009, 05:03 AM
David.C David.C is offline
Join Date: May 2009
Posts: 118
Default reply

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
Reply With Quote



Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Forum Jump

All times are GMT -4. The time now is 02:32 AM.
Powered by vBulletin
Copyright © 2000-2010 Jelsoft Enterprises Limited.
Copyright © 2009 NetBrain, Inc. All rights reserved.