Routing from unique IP address to two separate devices


New Member
Oct 22, 2022
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Hello everyone,

recently I started working with some PLCs who have Linux installed as an operating system. In general, I managed to solve the request from our customer, but I am looking for a way to improve things.

Our customer have a PC with an IP address of - on that PC a SCADA system is running which acts as a Modbus/TCP Client. It has to connect IP address on which a Modbus/TCP Server is running.

PLCs have IP addresses and and they act as a reduntant control solution (in case one of them stops or losses power or network connection, the other one takes over the process).

So, what I came up with is: when one of the PLCs is primary (controls the process), I add additional IP address ( using ip addr command and activate Modbus/TCP Server on that PLC. After that, I send a ping from to If this is a first time SCADA is connecting, it usually takes up to 7 seconds.

If there is a switchover to the backup PLC, on the primary PLC I delete the (to avoid having two same IP addresses on the network), I add this to the backup PLC and send a ping from it to SCADA. In the case of switchover, it takes around 17 to 20 seconds for this to take effect and SCADA will again connect to the IP

I would like to improve this switchover time to be as low ass possible.

Is there a way to add a routing option on both PLCs so that I don’t have to add and delete IP addresses, but to have PLCs listen to requests from to, and then to route this traffic from to both and

In that way, at a switchover I would just have to activate Modbus/TCP Server on the PLC which controls the process and that’s it.

Thank you for the support.



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/16 seems like an awful big subnet. Shrinking that will help.

Some distro's (mostly Redhat/Fedora based) will let you do something called NIC bonding or Teaming.
You can have two NIC with the same IP address. (Your switch has to support it).

You can set this up ans active/active or active/passive. If one side fails, the other side will keep going.
That way you don't even have to worry about failover to another IP address.

If you don't want to go through all that trouble, can you simply do a route add.