We have a number of Database severs running MongoDB on Debian plus a number of Application servers also on Debian. The db servers hold replicating db clusters, so they need to talk to each other. Application servers need to talk to all db servers (for reasons of fault tolerance). The servers are potentially spread across multiple hosting centers, so we need secure channels between all servers. The number of servers is bound to grow, so we need a VPN solution that's easy to maintain and expand. This is why I feel that SSH that we use for testing might not be up to the task and OpenVPN seems the way to go.

I have ruled out TAP, since I understand that this would mean all traffic going to all the servers - perhaps this is a misunderstanding and TAP acts more like a switch?

With TUN devices I imagine that all DB servers would live in their own separate subnet, they would also need a client configured to be able to connect to each of their peers. The application servers could live in a common subnet range with a client config only.

Does this sound like a reasonable setup? Strangely, on the web I did not find anything about multi-server with OpenVPN.

Thanks for all insights and ideas!