[c-nsp] Network Address Response

[Geoffrey Pendery]

No, I think he's asking why the router with address 10.0.0.5 responds
to pings that have a destination IP of 10.0.0.4.  The echo request is
targeted at a network address, not at the router.

I've also observed this behavior (more than ICMP though - I have a
router responding to SNMP and being discovered by our configuration
management team, on the network address of one of its interfaces) and
would like to know more about why...


-Geoff


On Fri, Jun 26, 2009 at 8:11 AM, Ms Geekgirl<msgeekgirl at gmail.com> wrote:
> 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.
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