PoE Part 1 – The fundamentals with a dip into some more complex topics, and PoE working in action and explained exhaustively!


*The image above is courtesy / cited by air-stream.org, so thanks to you guys!

This particular demonstration appears to be using an STP cable (Shielded Twisted Pair), however there are other alternatives that can be used, including foils and types of screens to keep the low voltage (hopefully) working properly.

As we know from CCNA, an Ethernet cable has 8 pairs / strands of cable, and 4 of them are used for Data Transmission. This is where the above image illustrates using the other the unused cables makes Power Over Ethernet possible!

Though most of them only use 2 of the cables for power, there is one type that uses all 4 unused cable pair, explained below JUST for the suspense ๐Ÿ™‚

In all seriousness though, I wanted to visually explain how the cable looks, and more importantly how it works with the leftover cable pairs so that is completely clear before reflecting upon all the different types of PoE.

With that being said – Lets learn some PoE stuff!

Firstly, I’d like to point out Cisco first made the PoE standard before the IEEE did, who then later made RFC’s for it and of course made it more efficient as they tend to do!

That being said, it was created out of necessity of being able to use a large amount of devices, and being able to power them. There weren’t enough wall plugs, technology was pumping out new hardware by the minute, and there needed to be a way to power this orchestra of chaos.

That is where PoE came in, and someone from Cisco first saw the potential of using the extra cable pairs to send low voltage over cables to power end devices like phones – This gave birth to Cisco Inline Power.

Cisco Inline Power uses CDP to negotiate with the remote device how much power to supply it, up to 7 watts. This was of course when Dinosaurs roamed the internet, and Cisco IP Phones were black and white / had 2 buttons / no sidecars / etc. This was also limited to Ethernet or FastEthernet ports on the switch, no Gig ports were supported.

Then as mentioned above, the IEEE put out some newer and improved standards, which I’ll throw up as some bullet points here:

  • 802.3af – Sometimes has a -2003 added to the name convention for the year it was created, this standard theoretically deliver 15.4 watts of power, although 2.4 watts gets lost in transmission. Does support Gig Ethernet.
  • 802.3at – Sometime has a -2009 added to the name convention for the year it was made as well, this standard is referred to as PoE+, as it is backwards compatible and provides up to 25.5 watts of power

To be clear, you may see those wrote as 802.af-2003 or 802.3at-2009, both are technically correct, I wouldn’t expect to see it on SWITCH but who knows.

  • The current reigning champion of PoE implementation is Cisco’s “Universal Power over Ethernet”, which can deliver up to a whopping 60 watts of Power over the wires, which if you didn’t guess already I will tell you – This uses all 4 unused cable pair whereas the previous PoE technologies only use 2 of the unused cable pair!

This is also known as “UPoE” and is not deployed on a very large scale, as only certain higher end switch models can run this type of PoE, so you will probably see on of the IEEE standards deployed out in the field.

Time for some CLI and demonstration of commands / what they verify!

I have a Cisco 7940 I bought off ebay but could never get it to work, because I only had older switches running Cisco Inline Power when I was studying the CCNA Voice track.

I shall now plug it in, and see if it gets stuck in registering but we can still see it!!! :

*Mar 1 01:15:28.078: %ILPOWER-7-DETECT: Interface Fa1/0/1: Power Device detected: Cisco PD
*Mar 1 01:15:29.135: %ILPOWER-5-POWER_GRANTED: Interface Fa1/0/1: Power granted
*Mar 1 01:15:32.641: %LINK-3-UPDOWN: Interface FastEthernet1/0/1, changed state to up
*Mar 1 01:15:33.648: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet1/0/1, changed state to up

Ha I have never seen this phone light up in my life!!! That is so cool, I am just going to enjoy that for just another moment here ๐Ÿ™‚


That is PoE right there, woo!!! I would really like to continue on either Collaboration (such a hot field all the time) or Wireless cause very few are Wireless “Experts”, so I am glad to see this seems to have no problem power a PoE device – That sight is beautiful ๐Ÿ™‚

So lets see what commands we need to see some stuff here, I unplugged Fa1/0/1 so it isn’t currently plugged into SW2, so I can leave a lot of the unnecessary output:

SW1#sh power inline

Module Available Used Remaining

(Watts) (Watts) (Watts)

—— ——— ——– ———
1 370.0 6.3 363.7
Interface Admin Oper Power Device Class Max
——— —— ———- ——- ——————- —– —-
Fa1/0/1 auto on 6.3 IP Phone 7940 n/a 15.4
Fa1/0/2 auto off 0.0 n/a n/a 15.4
Fa1/0/3 auto off 0.0 n/a n/a 15.4

So the table looks a bit odd but I’m too tired to try formatting it, however it shows the port Fa1/0/1 is drawing the 6.3 power needed for Cisco Inline Power, however if you look over to the far right that is not indicating the Maximum switch power it can provide but the “class” this device type belongs to which I will explain below!

So lets check out the CDP side of things:

SW1#sh cdp nei
Capability Codes: R – Router, T – Trans Bridge, B – Source Route Bridge
S – Switch, H – Host, I – IGMP, r – Repeater, P – Phone,
D – Remote, C – CVTA, M – Two-port Mac Relay

Device ID Local Intrfce Holdtme Capability Platform Port ID
SW3 Fas 1/0/7 179 S I WS-C3560- Fas 0/7
SIP000D6556F42F Fas 1/0/1 161 H IP Phone Port 1
R1 Fas 1/0/11 137 R S I 1841 Fas 0/1

Note that even though there is a P – Phone in the codes, but the phone shows up as a host, I assume that is because it acts almost like a mini-switch providing internet to your PC as well as itself.

This is how to verify on the switch where phones are plugged in (and other devices, as it will show routers / switches / wap’s / phones / etc) and running CDP.

I think it was taught at one time that “sh int trunk” would show phones on a switch, because their switchport had to be configured as a trunk, the phones port is an access port even though it also (usually) provides internet to the PC as well.

The magic of this is all taken care of through VLAN tagging (to be discussed later), however they stopped configuring these as trunks, as an IT Ninja could unplug your phone and plug it into their switch and just like that there is a rogue switch on the network!

So if you see “sh int trunk” in terms of phones, it is wrong, use “sh cdp nei” !

Now to talk about the Power Class and Power Max and how they correlate

There are 5 classes available on a switch, however with “sh inline power” they all say n/a, and this is because they are all in Power Class Zero and I shall explain bullet point style:

  • Power Class 1 – Max 4.0 Watts
  • Power Class 2 – Max 7.0 Watts
  • Power Class 3 – Max 15.4 Watts (Power Class Zero IF n/a is under Class column)
  • Power Class 4 – Max 30 Watts

Hence, 5 classes of power,

So I’ll repeat just the important stuff from “sh power inline” to explain this concept:

Interface Admin Oper Power Device Class Max
——— —— ———- ——- ——————- —– —-
Fa1/0/1 auto on 6.3 IP Phone 7940 n/a 15.4
Fa1/0/2 auto off 0.0 n/a n/a 15.4
Fa1/0/3 auto off 0.0 n/a n/a 15.4

All Power Class Zero, with Port 1 drawing 6.3 watts of Power, and most of the columns are self-explanatory (I hope) like Oper (on) Power (6.3) Device (IP Phone 7940) – However there is one column in their called “Admin” that will be Part 2 of PoE.

I’d keep going, but I might pass out on the keyboard, and the Admin “Auto” is what is driving some of these other values so we better know what they are (and how to change them to cause all sorts of chaos).

Until next time!


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