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Power Over Ethernet (PoE) Operation
Power Availability and Provisioning
Power Availability and Provisioning
Powered Device (PD) Support
The switch must have a minimum of 15.4 watts of unused PoE power available
when you connect an 802.3af-compliant PD, regardless of how much power
the PD actually uses.
Note that once a PD connects to a port and begins operating, the port retains
only enough PoE power to support the PD's operation. Unneeded power
becomes available for supporting other PD connections. Thus, while 15.4
watts must be available for the switch to begin supplying power to a port with
a PD connected, 15.4 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 the switch,
you can connect one new PD without losing power to any currently connected
PDs. If that PD draws only 3 watts, then 17 watts remain available and you can
connect at least one more PD without interrupting power to any other devices.
If the next PD you connect draws 5 watts, then only 12 watts remain unused.
With only 12 watts available, if you connect yet another PD, the lowest-priority
port will lose PoE power until the switch once again has 15.4 or more watts
available. (For information on power priority, refer to "Power Priority" on
page 11-5.)
Disconnecting a PD from a port causes the switch to stop providing PoE power
to that port and makes the power available to other ports configured for PoE
operation. If the PoE demand becomes greater than the available power, the
switch transfers power from lower-priority ports to higher-priority ports.
(Ports not currently providing power to PDs are not affected.)
Note
15.4 watts of available power is required for the switch to begin delivering
power to a port, such as when a newly connected PD is detected or when
power is released from higher priority ports. Depending on power demands,
lower-priority ports on a switch with high PoE power demand may
occasionally lose power due to the demands of higher-priority ports. (Refer
to "Power Priority" for further details.)
In certain cases up to 17W of power may be reserved by the switch.
11-4
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