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I currently have multiple servers which are all on their own separate networks (each server on a different network talking to other devices). I want to implement a common central data server which will store database backups for the SQL instances on the servers. My plan right now is to add a router to the network architecture which will allow the servers to communicate with the Central data server. In the past these servers were all on the same network which caused communication problems, so they were separated. This leads to my question: If the now "isolated" server networks are connected to the same router and all communicate with a central data storage unit, will this keep them "isolated" or will it effectively be putting all the devices back on the same network?

For context, since they were networked in the past, they're all on the same subnet. The servers all communicate to field devices such as PLC's and HMI's and exist in a manufacturing environment.

Thanks in advance!

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    I'm curious: what were the "communications problems" you experienced? How did moving them the separate subnets fix that? Do you have some sort of filtering between the subnets? – Ron Trunk Dec 1 '16 at 19:38
  • How do you currently have them "isolated" if they are all configured in the same subnet? – John K. Dec 1 '16 at 20:10
  • They don't communicate outside of the system. Currently for each system, the server talks to only the controls equipment and the operator display PC's. Nothing else. They are physically not wired to anything else. And to answer Ron... I wasn't around for this issue, but according to the customer, the system would operate so slowly that all functionality would hang and some network nodes would even lose connectivity completely. They didn't have specific information outside of that explanation. Apparently physically separating the systems alleviated these issues. – AtLeastITried Dec 1 '16 at 20:21
  • Some PLC equipment flood broadcast packets as their way of communication. Every device on the subnet receives these broadcasts and must process them, so devices that receive packets from other control groups which they aren't a part of still have to process them. If this was the original problem it can be solved with a router since the router will break up the broadcast domains. It will involve configuring a separate subnet for each of these PLC groups though. – John K. Dec 1 '16 at 20:46
  • Also, there is a Cisco validated design for manufacturing networks that you can take a look at: cisco.com/c/en/us/td/docs/solutions/Verticals/CPwE/CPwE_DIG/… – John K. Dec 1 '16 at 20:49
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Routers route between networks. Each router interface is a separate network. Each router interface needs to be a different network (subnet), otherwise you will need to set up some sort of NAT for each network.

I doubt you would consider your PC to be on the same networks as Google, even though your PC contacts Google by a router connection. So, yes, your servers will be on different networks, even if those networks are connected with a router.


It really sounds like you need to implement QoS. Without QoS, your server backup traffic will have the same priority as production traffic, and that can cause production problems. QoS is all about fairness, as you define it, and the need to separate your servers onto different networks really means you should probably redesign the network and implement a good set of QoS policies. Providing enough bandwidth, taking into account things like bandwidth oversubscription ratios, and properly sharing it, you could have a lot of servers on the same network.

  • I suspect his "communications problems" is something other than bandwidth. – Ron Trunk Dec 1 '16 at 19:39

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