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11-8
Power Over Ethernet (PoE) Operation
General PoE Operation
N o t e
The ports on a PoE module support standard networking links and PoE links.
Thus, you can connect either a non-PoE device or a PD to a PoE-enabled port
without reconfiguring the port.
PD Support
When you connect the first PD to a PoE port, the PoE module must have a
minimum of 17 watts of PoE power available in order to detect and supply
power to the device. Power is allocated dynamically among PoE modules, with
each PoE module allocated a minimum of 22 watts of PoE power—17 watts
for detection and an additional 5 watts for variations in any applied power
loads. To best utilize the allocated PoE power, spread your connected PoE
devices as evenly as possible across modules.
Depending on the amount of power the power supply device delivers to a PoE
module, there may or may not always be enough power available to connect
and support PoE operation on all 24 Gig-T ports in the module. When a new
PD connects to a PoE module and the module 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.
Note that once 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. Thus, 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 currently connected PDs on that module. If that PD draws
only 3 watts, then 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, then
only 12 watts remain unused. With only 12 unused watts available, if you then
connect yet another PD to a higher-priority PoE port, then the lowest-priority
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