Power over Ethernet (PoE)

Notes on Power over Ethernet, comparing the three power source options, PoE standards and wattages, and the power inline configuration commands.

Power can come from the following three sources:

1. An external AC adapter connected directly to the device:

• The external AC adapter plugs into a normal AC wall outlet and provides 48V DC to the device. These adapters, commonly called wall warts, are handy if no other power source is available. However, if a power failure occurs in the room or outlet where the adapter is located, the powered device will fail.

2. A power injector, which connects to AC power near an Ethernet switch and provides DC power over the network data cable:

• If a device consumes more power than a switch provides, then we use a PoE injector. Typically, a power injector is connected to AC power in a wiring closet close to the switch. One pitfall of using power injectors is that you need one injector and one AC power outlet per switch port!

3. A switch capable of providing DC Power over Ethernet (PoE) over the network data:

• More elegant solution. Here, a 48V DC supply is provided to a device over the same unshielded twisted-pair cable that is used for Ethernet connectivity. The DC power source is the Catalyst switch itself. No other power source is needed unless an AC adapter is required as a redundant source.

PoE Methods

• Cisco Inline Power (ILP): 7W
• IEEE 802.3af (PoE): 15.4W
• IEEE 802.3at (PoE+): 25.5W
• Cisco Universal PoE (UPoE): 60W

Configuration

Switch(config)#interface gigabitethernet1/0/1
Switch(config-if)#power inline {auto [max milli-watts] | never | static [max milli-watts]}

• auto sets the interface to automatically detect and supply power to the powered device. This is the default configuration.
• static sets the interface to higher priority than auto.
  • If necessary, you can use the max keyword to specify the maximum wattage allowed on the interface (4000 to 15400 milliwatts).
• never to disable detection and power for the PoE capable interface.

Verification

Switch#show power inline fastethernet 4/1
Available:677(w) Used:11(w) Remaining:666(w)
Interface Admin Oper Power(Watts) Device Class
 From PS To Device
--------- ------ ---------- ---------- ---------- ------------------- -----
Fa4/1 auto on 11.2 10.0 Ieee PD 0
Interface AdminPowerMax AdminConsumption
 (Watts) (Watts)
---------- --------------- --------------------
Fa4/1 15.4 10.0