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From: crzzy1 on 23 Apr 2010 09:15
On Apr 21, 1:42 pm, Rob <nom...(a)example.com> wrote:
> crzzy1<cozz...(a)hotmail.com> wrote:
>
> > Perhaps someone can put this into perspective for me.
> > This is an age old question that has arisen many times throughout the
> > years and my web searches have failed to answer this.
> > The host is visible in arp and shows incomplete, the host cannot be
> > pinged.
> > Sometimes the prob is a cable, sometimes a host route setting,
> > sometimes an intermediary device.
> > (Let me know if there are other known things that cause this).
>
> > ca-santa-barbara-router#sh arp | i 70.169.191
> > Internet  71.169.191.209          0   Incomplete      ARPA
> > Internet  71.169.191.218          -   0018.731f.407d  ARPA   Ethernet1
>
> > My question is, how is it, that we get the IP address from the
> > directly connected host into the ARP cache, but not the MAC address?
> > We arp out for that IP, and the connected host is smart enough to
> > reply that it has that IP, but it isn't smart enough to send it MAC
>
> No, this just means the cisco device has sent an ARP for the IP
> in the list (71.169.191.209) and it has not received a reply.
>
> So the host is down or not connected.

That is not correct. I forgot to mention that this is a /24,
and only the host 71.169.191.209 is in fact connected.
I can ping the broadcast address, and only this IP comes into the ARP
table.
In this case there was a problem with the routing on the host.
So in other words, my router sees the host, the host in some way
replies that it has this IP, but is unable to say what its MAC is.
I have seen this numerous times, but never have figured out what
really takes place to let my router know that that host is there, but
not what is the MAC?

crzzy1


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