On Thu, Jun 25, 2009 at 4:28 PM, Ray Burkholder<[email protected]> wrote: > I was wondering the reasoning for routers/switches to respond for the > network portion of an ip-address range. > > For example, a router interface A with 10.0.0.1/30 and interface B with > 10.0.0.5/30. > > Generate a ping from a device several hops away on the A side to the B side > network address of 10.0.0.4. The router will respond with an echo reply > with an address of 10.0.0.1. > > Is this expected behaviour? And the reason? > > Ray
Yes. By default. you will almost always get a response from the closest interface to the source of the ping (*unless instructed otherwise in each hop's configuration.) In your example, this is what that looks like. Somewhat simplistic and others may have a better response, but here goes... The echo-request is sent from c1 to the IP 10.0.0.5 assigned to intB on dest-rtr. dest-rtr will receive the echo-request on intA, forward to intB. dest-rtr will lookup the best return route to your network. The return route chosen is towards hop2, via intA and the packet is sent out through intA. | > > > > > path of echo request > | ^ v [c1-B]----[A-hop1-B]----[A-hop2-B]----[(intA) dest-rtr (intB)]----[A-c2] ^ v | < < < path of reply < < | If you were to ping c2, the response would come from c2's IP (since this node only has one IP and is the closest to you :) In anticipation of a possible traceroute question, the same applies. If you were to trace to c2, the responses* would all come from the closest interface towards c1. In the above, all the responses would come from the _A_ interfaces of each hop. If c2 were to ping/trace to c1, the responses would come from the _B_ interfaces of each hop. I hope that I haven't confused anything and that this was helpful for you. - - - mgg Like a seedling in the Spring, green and vulnerable. _______________________________________________ cisco-nsp mailing list [email protected] https://puck.nether.net/mailman/listinfo/cisco-nsp archive at http://puck.nether.net/pipermail/cisco-nsp/
