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Power Over Ethernet (PoE) Operation
PoE Operation
Term Use in this Manual
priority class Refers to the type of power prioritization that uses Low (the default), High, and Critical
priority assignments to determine which groups of ports will receive power. Note that power
priority rules apply only if PoE provisioning becomes oversubscribed.
PSE Power-Sourcing Equipment. A PSE provides power to IEEE 802.3af-compliant or IEEE
802.3at-compliant PDs directly connected to the ports on the module. The PoE module is an
endpoint PSE.
RPS Redundant Power Supply. An RPS device provides power to a switch if the switch’s internal
power supply fails. RPS power does not provision PoE ports on a switch whose internal
power supply has failed. See also “EPS” above.
PoE Operation
Powered Device (PD) Support
The switch must have a minimum of 17 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 17 watts
must be available for 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 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 17 or more watts
available. (For information on power priority, refer to “Power Priority
Operation” 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.)
11-5