Lose net access on replugging -- why?

I'm curious to know what's happening here. It's not a fatal problem,
but it's annoying. [Probably not Linux specific either, but that's
where I'm hitting it.)

We have two machines with the same IP address (not both online at the same
time, obviously!) while I build up a new server to replace the current one.
If I unplug the old machine from the net, and plug in the new one, the IP
address is no longer available -- ping no longer responds for example.

Doing things like ifconfig eth0 down/up don't fix the situation. I have
to reboot the connected machine to regain access. (The same thing happens
once I have the new one working and want to go back to the old one.)

If I just unplug one machine and then plug it back in after a few minutes
I don't lose the connection. So what's the cause? I assume it's to do
with the different MAC addresses, but how?

-- Pete --

Pete Goodeve <(E-Mail Removed)> wrote:
> If I just unplug one machine and then plug it back in after a few
> minutes I don't lose the connection. So what's the cause? I assume
> it's to do with the different MAC addresses, but how?

ARP caches. Some hardware (IIRC Cisco routers are known for this)
will place a limit on how soon they will update an entry in their ARP
cache. Until that happens, they will still try to send traffic to the
old MAC.

Simialrly, some systems may not take immediate notice of a gratuitous
ARP generated upon an interface being brought-up. You end-up waiting
for their ARP refresh intervals to pass before they re-ARP for the
IP/MAC mapping and get the new MAC associated with the IP.

rick jones
--
denial, anger, bargaining, depression, acceptance, rebirth...
where do you want to be today?
these opinions are mine, all mine; HP might not want them anyway...
feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...

In article <OoPAg.1603$(E-Mail Removed)>,
Rick Jones <(E-Mail Removed)> wrote:
>Pete Goodeve <(E-Mail Removed)> wrote:
>> If I just unplug one machine and then plug it back in after a few
>> minutes I don't lose the connection. So what's the cause? I assume
>> it's to do with the different MAC addresses, but how?
>
>ARP caches. [.........]

Thanks for the explanation. I guess rebooting forces a cache refresh?

(Though I'm not sure I completely understand... These machines (pinger
and pingee) are all on the same subnet, so shouldn't the packets just
arrive regardless?)
-- Pete --

(E-Mail Removed) (Pete Goodeve) writes:


>I'm curious to know what's happening here. It's not a fatal problem,
>but it's annoying. [Probably not Linux specific either, but that's
>where I'm hitting it.)

>We have two machines with the same IP address (not both online at the same
>time, obviously!) while I build up a new server to replace the current one.
>If I unplug the old machine from the net, and plug in the new one, the IP
>address is no longer available -- ping no longer responds for example.

Of course not. IP addresses are NOT how a machine is addressed on the local
net. There is no way that a packet can be delivered by IP address alone.
They are delivered on the local net via MAC addresses. So a packet gets
sent to your IP , at your gateway into your net, there is a translation
table (ARP cache) which translates IP to MAC and the packet is sent on that
last leg via the MAC address. When you change computers you change ethernet
cards and MAC addesses.

>Doing things like ifconfig eth0 down/up don't fix the situation. I have
>to reboot the connected machine to regain access. (The same thing happens
>once I have the new one working and want to go back to the old one.)



>If I just unplug one machine and then plug it back in after a few minutes
>I don't lose the connection. So what's the cause? I assume it's to do
>with the different MAC addresses, but how?

> -- Pete --

(E-Mail Removed) (Pete Goodeve) writes:

>In article <OoPAg.1603$(E-Mail Removed)>,
>Rick Jones <(E-Mail Removed)> wrote:
>>Pete Goodeve <(E-Mail Removed)> wrote:
>>> If I just unplug one machine and then plug it back in after a few
>>> minutes I don't lose the connection. So what's the cause? I assume
>>> it's to do with the different MAC addresses, but how?
>>
>>ARP caches. [.........]

>Thanks for the explanation. I guess rebooting forces a cache refresh?

>(Though I'm not sure I completely understand... These machines (pinger
>and pingee) are all on the same subnet, so shouldn't the packets just
>arrive regardless?)

Arrive where? ALL delivery along a subnet is by MAC, not IP. When y ou type
in an IP your computer if it is on the same subnet must look up its arp
cache to figure out to what MAC address to deliver the packet. Thus you
need to fluch the arp cache on all of the machines on the subnet.


> -- Pete --

In comp.os.linux.networking Pete Goodeve <(E-Mail Removed)>:

> I'm curious to know what's happening here. It's not a fatal problem,
> but it's annoying. [Probably not Linux specific either, but that's
> where I'm hitting it.)

> We have two machines with the same IP address (not both online at the same
> time, obviously!) while I build up a new server to replace the current one.
> If I unplug the old machine from the net, and plug in the new one, the IP
> address is no longer available -- ping no longer responds for example.

> Doing things like ifconfig eth0 down/up don't fix the situation. I have
> to reboot the connected machine to regain access. (The same thing happens

MAC addresses used for subnet routing of packages as others
already pointed out. I suggest to fix/reconfigure your network
(switches) equipment. Doing this regularly, I just restart the
network (with prepared new settings) and services that might need
it as soon as the "old" system doesn't answer anymore on a
running ping during shutdown. Works without any problem or
reboot. Remote console access eases things up if you aren't sure
what you are doing, or/and unexpected problems pop up...

Good luck

--
Michael Heiming (X-PGP-Sig > GPG-Key ID: EDD27B94)
mail: echo (E-Mail Removed) | perl -pe 'y/a-z/n-za-m/'
#bofh excuse 383: Your processor has taken a ride to Heaven's
Gate on the UFO behind Hale-Bopp's comet.

Pete Goodeve wrote:
> In article <OoPAg.1603$(E-Mail Removed)>,
> Rick Jones <(E-Mail Removed)> wrote:
>> Pete Goodeve <(E-Mail Removed)> wrote:
>>> If I just unplug one machine and then plug it back in after a few
>>> minutes I don't lose the connection. So what's the cause? I assume
>>> it's to do with the different MAC addresses, but how?
>> ARP caches. [.........]
>
> Thanks for the explanation. I guess rebooting forces a cache refresh?
>
> (Though I'm not sure I completely understand... These machines (pinger
> and pingee) are all on the same subnet, so shouldn't the packets just
> arrive regardless?)
> -- Pete --
>

Not if you're going through a switch. They cache which mac is on which
port.

Sometimes sending data out from the newly online machine will help
everyone notice that the mac has changed. It varies by OS.