I am working with POE powered devices, and one use case we see is that our device is connected to a POE injector, but behind the POE injector, there is a POE switch that can do LLDP negotiation.
(The thing here is that a POE injector does not normally support POE software negotiation (LLDP), and that it does not have any IP address.)
According to the POE standard, after the boot sequence the DUT should check for LLDP support, and try to do fine grained negotiation using POE-LLDP.
So, if doing this the simplest possible way, what can happen is that the device receives 25.5W through hardware negotiation (from the POE injector), then the DUT connects to LLDP (in the switch) which might tell it that it has only received 12.95W, (since that is what the POE switch can provide).
Then the DUT suddenly may think it has too little power to work, and shut down.
Also, since the 802.3at (and I assume bt) standard seem to be a bit relaxed in what it allows when it comes to hardware versus software negotiation it is not clear to me how these use cases should be handled.
Recommending that customers do not connect a POE switch behind a POE injector is already done I assume.
We were discussing that if the LLDP signalling indicates a lower power than what we have already received though hardware negotiation, we might discard the LLDP signalling as it is probably from the wrong node, but how did the ones who wrote the specifications think this should be handled?
Are there other recommendations outside the specification on how this should be handled?
Is there any reliable (or at least sometimes helpful) way to detect when a POE switch is connected behind a POE injector?