SolarWinds offers a product called SolarWinds HA that allows you to have an 'active-passive' cluster for NPM or any other SolarWinds modules that you have installed.
This can be installed with both servers (active and passive) in the same subnet, or in different subnets. If both servers are in the same subnet, then you will use a Virtual IP address (VIP), whereas when both servers are not in the same subnet, you will have to update a DNS record.
We recommend, if possible, to use both servers in the same subnet, it is easier to configure, to maintain, and you will generate less traffic (you don't have to duplicate netflow, syslog, traps,...).
If you have any other question, please let me know.
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