[c-nsp] Network Address Response

[Ms Geekgirl]

On Thu, Jun 25, 2009 at 4:28 PM, Ray Burkholder<ray at oneunified.net> 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.