X20 CPU redundancy configuration


Our customer is designing network for Redundancy configuration.
Could help

check out attachment and clarify on few questions below?

  1. Can we have 2 subnets for each PLC?
  2. I see cluster IP in configuration, Can we polling modbus or access data by OPCua via cluster IP Address? Please also explain about cluster IP in the redundancy controller ?
  3. Can we communicate with Secondary PLC in case the Primary still running?
  1. I have not tested this on real HW but at least Automation Studio does not output an error or a warning when IP addresses are configured as your attachment shows.

  2. You can use Cluster IP for communication, this will always connect to the active PLC. However during a redundancy switchover there might be a need to re-establish the communication so you might experience a momentary loss of connection. And not all clients can automatically re-establish the communication, so you need to test this with you setup.

  3. You can communicate directly with each PLC when you use the direct IP address off the PLC so you can have simultaneous separate connections to both PLCs at the same time.

You can read more about the limitations from this page.

Thanks for the reply and support. We will find out more from the help file on the limitation of the redundancy CPU.

Hi Tommi,

in the help function it says both primary/active or secondary/inactive PLCs are seen as one from outside. But at the same help file it also says that, “Operating principle It is also possible to access one of the two rCPUs selectively via the primary and secondary connection settings.”

Below is the link that mentioned


Thank you
Best regards

Hi Jai,

When using the cluster IP address the PLCs are seen as one(Point 2 in my previous reply).

So if you use for example Automation Studio to connect to the IP address (Cluster address) this would always connect to the Active PLC.

When using the independent IP addresses, they can both be connected indepently (Point 3 in my previous reply).

If you use you would always connect to the primary PLC (if possible) and would always connect to the secondary PLC (if possible). Which PLC is the active one does not play a role here as you are connecting directly.

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Hi Tommi,

Thank you for a clear explanation. appreciate it.

Best regards and have a great day.
Jai Kumaran