PD support

To best utilize the allocated PoE power, spread your connected PoE devices as evenly as possible. Depending on the amount of power the power supply device delivers to a PoE switch, there may or may not always be enough power available to connect and support PoE operation on all the ports. When a new PD connects to a PoE switch and the switch does not have enough power left for that port:
  • If the new PD connects to a port “X” having a higher PoE priority than another port “Y” that is already supporting another PD, then the power is removed from port “Y” and delivered to port “X”. In this case the PD on port “Y” loses power and the PD on port “X” receives power.

  • If the new PD connects to a port “X” having a lower priority than all other PoE ports currently providing power to PDs, then power is not supplied to port “X” until one or more PDs using higher priority ports are removed.

In the default configuration (usage), when a PD connects to a PoE port and begins operating, the port retains only enough PoE power to support the PD's operation. Unused power becomes available for supporting other PD connections. However, if you configure the poe-allocate-by option to either value or class, all of the power configured is allocated to the port.

For PoE (not PoE+), while 17 watts must be available for a PoE module on the switch to begin supplying power to a port with a PD connected, 17 watts per port is not continually required if the connected PD requires less power. For example, with 20 watts of PoE power remaining available on a module, you can connect one new PD without losing power to any connected PDs on that module. If that PD draws only 3 watts, 17 watts remain available, and you can connect at least one more PD to that module without interrupting power to any other PoE devices connected to the same module. If the next PD you connect draws 5 watts, only 12 watts remain unused. With only 12 unused watts available, if you then connect yet another PD to a higher-priority PoE port, the lowest-priority port on the module loses PoE power and remains unpowered until the module once again has 17 or more watts available. (For information on power priority, see Power priority operation.)

For PoE+, there must be 33 watts available for the module to begin supplying power to a port with a PD connected.

Disconnecting a PD from a PoE port makes that power available to any other PoE ports with PDs waiting for power. If the PD demand for power becomes greater than the PoE power available, power is transferred from the lower-priority ports to the higher-priority ports. (Ports not currently providing power to PDs are not affected.)