Power over Ethernet is pretty much what it sounds like. It allows Power Sourcing Equipment (PSE) to supply power to Powered Devices (PD) over an Ethernet cable. Typically, the PSE is a PoE-capable switch, and PDs are IP Telephones, wireless access points, or any number of other devices.
Note
In PoE power is delivered over the same cable that transmits data. You don’t need a second cable for power.
PoE can simplify deploying and maintaining network devices and cabling by eliminating the need for some or all local power sources. While most workstation will have power outlets, they may not have enough to plug in every device that needs power. If the telephone can get its power through its network cable, then that’s one fewer wall outlets needed.
Supplying Power
Most electronic devices need their electricity supply to fall within a particular range to work properly. Too little and they won’t turn on, or may not behave correctly. Too much and they may be damaged or destroyed.
Therefore, it is important that PSEs supply enough power to devices, and never supply too much. For this reason PoE has a process for determining if a connected device needs power, and how much. When a device is first connected, the PSE sends low-power signals and monitors responses to determine the device’s power needs (if it has any). If the device needs power, the PSE supplies power so the PD can boot then continues to monitor the PD and supply the required amount of power.
Power Policing
Power Policing can be configured to prevent a PD from taking too much power. This is beyond the scope of the CCNA, but here are some of the basic commands for configuring PoE Power Policing:
power inline police- Configures power policing with the default settings (disable the port and send Syslog message if a PD draws too much power)
- Equivalent to
power inline police action err-disable - Interface will be put in ‘error-disabled’ state. It can be re-enabled with
shutdownthenno shutdown
power inline police action log- Doesn’t shut down the interface if the PD draws too much power. It will restart the interface (causing the PD to reboot) and send a Syslog message.
Standards
There are various current industry standards for PoE, though the concept was initially implemented for commercial use by Cisco as Cisco Inline Power (ILP).
| Name | Standard # | Watts | Powered Wire Pairs |
|---|---|---|---|
| Cisco Inline Power (ILP) | Cisco-proprietary (not standard) | 7 | 2 |
| PoE (Type 1) | 802.3af | 15 | 2 |
| PoE+ (Type 2) | 802.3at | 30 | 2 |
| UPoE (Type 3) | 802.3bt | 60 | 4 |
| UPoE+ (Type 4) | 802.3bt | 100 | 4 |