If I recall correctly, NTA licenses the flows not devices or interfaces. If you have NTA collecting 20 flows from the same router / switch than that would take 20 licenses.
NPM and NTA licensing have to match. If you have NPM 2000, you have to buy NTA 2000.
You can change the settings in NTA to collect flows from any devices, then add then in the same settings screen, however, usually you monitor the interfaces on the router / switches in NPM as well.
That's not correct. Your NTA license is based on your Orion NPM license size - this is best approximation we have for the traffic analysis value received since not all interfaces are created equal (e.g. WAN interface versus internal switch interface).
So, your NTA license allows you to collect NetFlow from any of the interfaces monitored in Orion NPM. Orion NPM interface-based licensing is your only limiting factor.
Hope this helps clarify. If not, please let me know and I'd be happy to have a call to walk through this in more detail.
Are there any plan to change this licensing model? Not all devices support netflow, so in our case it seems extreme to have to pay for NTA2500 when we can only monitor the interfaces on the 10 devices that do netflow/sflow (total of 172 interfaces).
Shouldn't NTA be licensed like APM?