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I have a small LAN. there are:

  • One linux PC with IP x.x.x.10
  • an unmanaged switch
  • two different devices that share the same IP (x.x.x.1) (lets call them device 1 and device 2).

The idea is to power up device 1, then linux PC connects to it using modbus tcp/ip protocol, then disconnects and the device is powered down.

Next step is that device 2 is powered up. linux PC connects to it with modbus tcp/ip, changes its IP address to (x.x.x.2) then disconnects and after that device 2 is powered down.

So I end up with two devices with different IPs that can work simultaneously.

My problem is that when I try to connect to device 2 it takes more than one minute to establish the connection. I think the problem is MAC learning. Is there any way to overcome this?

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  • Unfortunately, you have an unmanaged switch, which is off-topic here as there is nothing to configure, and your PCs are off-topic, as is Modbus on TCP because it is a protocol above OSI layer-4. As Zac explains, it is a host problem with ARP, not a network problem.
    – Ron Maupin
    Jul 13 at 12:39
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MAC learning on the switch happens 'instantly' - as soon as the first frame is received by the switch. I don't think that is a problem, especially since multiple MAC addresses on a single port are no problem generally.

But: any device trying to send to the duplicate IP address is likely to have the previous MAC in its ARP cache though - make sure you delete that entry/clear the cache to force an update and then reconfigure the device.

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