Power Over Ethernet

By | March 11, 2020

PoE stands on power over Ethernet. It is used to provide the DC power over Ethernet cable through switch port. it can provide maximum 48 V DC over Ethernet cable that is applied from a power capable switch. A switch can offer the power to IP phones or to any powered device that can request and use the PoE power.

To configure the PoE , platform must be comparable and should have two power supply units to fulfill the additional power requirement. PoE feature is not present on all Cisco switches. So to use this feature , switch should be a PoE enabled switch like 3750 PWR , 4500 and 6500.

A switch always keeps the PoE disabled when switch port is down. But it continuously try to detect a powered device connected on port.

Two PoE methods are available : Cisco inline power and IEEE 802.3af ( Open Standard )

Cisco Inline Power ( ILP )

Inline power is an Cisco proprietary feature. A inline power enabled switch sends a 340 Khz test tone on transit pair of Ethernet cable and expects to receive looped back tone.

It transmits the 48 V DC power over 2nd and 3rd paris of data cable ( RJ45 Pin 1,2 & 3,6 ). It uses CDP messages to discover the power requirement of receiving powered device.

By default power budget is 15.4 W and we can change it from 11.00 to 15.00.

After Apply the power to a switch port , Switch waits for 4 seconds to detect the powered device. If powered device is not present then power will be removed from port.

After removed a IP Phone ( Powered Device )from a PoE enabled port , We should not connect a normal device ( None powered device ) on the same port. We should wait for minimum 10 seconds before connect a normal device on that port.

Below is the example for enable the PoE on a switch port:

Switch#conf t
Switch(config)# interface f0/0
Switch(config-if)# power inline

Below is the syntax for above command

Switch(config-if)# power inline { auto [ Max mili-watts ] / static [ max mili watts ] / never}

Never keyword is used to disable the PoE on a switch port.

IEE 802.3af

This open standard method of PoE and alternate for ILP. A 802.3af enabled switch begins by supplying a small voltage on its switch ports and expects the resistance of 25 Ohm , if powered device is present.

It can use five types of power classes to test the corresponding resistance value or device’s power requirement.

If power class discovery is not supported by powered device , then default class of 15.4 W will be use. It can use 2nd and 3rd pair ( RJ45 pin 1,2 & 3,6 ) or 1st and 4th pair ( RJ45 pin 4,5 & 7,8) of data cable.

Power Classes

Power ClassMaximum Power Offered at 48 V DCDescription
015.4 WDefault Class
14.0 WOptional Class
27.0 W Optional Class
315.4 W Optional Class
4ReservedReserved

Show commands

Switch# show power inline
Switch#show power inline f0/0

Debug Commands

Switch#debug ilpower controller
Switch#debug cdp packets

Below is the image of Cisco 2960-24PC-L, 24 x POE 

cosco poe switch

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