I am asking if anyone has tried to take one (or more) POE ports from the back of an NVR, plug them into a managed switch using discrete VLANs for each, connected to a POE powered camera across a distance greater than several hundred meters. I can't be the first.
A Hikvision NVR will only 'find' a single camera when multiple cameras are connected to a PoE switch that is uplinked to a NVR PoE port.
I've tried this myself, it's how the firmware is designed to interact with the hardware.
People have done this on a Dahua NVR with better, but variable and unpredictable results, based on some recent posts.
I've tried the same on a QVIS NVR where the PoE ports were simply a switch hooked up to the LAN port, not running a separate IP segment like the Dahua or Hikvision NVRs.
The PoE-port-connected cameras all manifest as LAN-connected.
Which is how you'd be able to extend the cameras distance as far as you want by daisy-chaining normal switches from the LAN to eventually a PoE switch.
So whether you can use a switch almost as a midspan PoE injector varies with the design of the NVR in question.
The potential issue I foresee is weather or not the NVR POE ports are auto sensing.
Auto-sensing for PoE action - or link speed?
The brands of NVR I alluded to above all had 802.3af PoE - so the supply of power is by the sensed handshake.
And Dahua and Hikvision have a configuration option in their PoE port settings to drop the normally auto-sensed link speed down to 10Mbps full duplex and extend the supported cable length up to around 300m.