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 OperationPowered 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