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 on page 87.)
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.)
Power priority operation
If a PSE can provide power for all connected PD demand, it does not use its power priority settings to allocate
power. However, if the PD power demand oversubscribes the available power, the power allocation is prioritized
to the ports that present a PD power demand. This causes the loss of power from one or more lower-priority ports
to meet the power demand on other, higher-priority ports. This operation occurs regardless of the order in which
PDs connect to the module's PoE-enabled ports.
Power allocation is prioritized according to the following methods:
• Priority class methodAssigns a power priority of low (the default), high, or critical to each enabled PoE port.
• Port-number priority methodA lower-numbered port has priority over a higher-numbered port within the same
configured priority class, for example, port A1 has priority over port A5 if both are configured with high priority.
Configuring PoE operation
Disabling or re-enabling PoE port operation
Syntax:
[no] interface <port-list> power-over-ethernet
Re-enables PoE operation on <port-list> and restores the priority setting in effect when PoE was disabled on
<port-list>.
The no form of the command disables PoE operation on <port-list>.
Default: All PoE ports are initially enabled for PoE operation at Low priority. If you configure a higher priority, this
priority is retained until you change it.
NOTE:
For PoE, disabling all ports allows the 22 watts of minimum PoE power or the 38 watts for PoE+
power allocated for the module to be recovered and used elsewhere. You must disable ALL ports for
this to occur.
Enabling support for pre-standard devices
The HPE switches covered in this guide also support some pre-802.3af devices. For a list of the supported
devices, see the FAQ for your switch model.
Chapter 4 Power Over Ethernet (PoE/PoE+) Operation 87
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