Cisco POE

Switches that work with "Cisco Inline Power" devices (Pre-standard/Legacy POE)

Cisco 200 & 300 series Small Business Switches
  • Must be running Firmware version or later.
  • EXCEPT: SG200-08P and 200E Switches. These switches use a different chipset.
  • More information is available HERE and HERE

Netgear FSM7326P
  • Work out of the box but are NLA. They do pop up on eBay and elsewhere used.

How to use Standard 802.3af Equipment w/ Cisco 7940/60 G

Most of this thread needs to be erased because a lot of the information is just plain wrong. I considered correcting all of the individual posts but realized that would be futile so I'm simply going to repost all of the relevant facts.

CDP is not relevant. Your switch does not need to support CDP and the phone will work whether or not it is present. From Cisco's own web site:

"For Cisco pre-standard PDs (Powered Devices), if CDP is enabled on the switch, 15.4W is initially allocated, and then further refined when the CDP message is received from the PD. If CDP is disabled on the switch, or if the PD does not support the Power requirements field of the CDP message, the initial allocation value of 15.4W is used throughout the duration of the connection."

In other words: CDP is only used to reduce the power allocation on those switches that support it. If the switch doesn't support CDP it simply supplies full power and the phone doesn't care.

Cisco Pre-Standard phones can accept power on either the spare pairs OR the data pairs.

The 3com power splitters may or may not work. The 3com power splitter will only work with Type B PSE's that supply power on the unused data pairs. The 3com part is simply a splitter- it has no way of pulling power off the data lines (as is the case with some 802.3af PSE's) and splitting it out as that would require magnetics (transformers) that are not present in this part. This cable does not work with an NetGear FS108P, for example. I tried it with several different 7940's and 7960's.

How Cisco Pre-Standard POE Works:

Cisco phones have a low-pass filter (inductors and capacitors) connecting the different cable pairs on the phone. Cisco switches send a low frequency signal down one pair, and if they get the same signal back on another pair, they know there is a low pass filter present and that the device connected wants power. At this point the switch supplies power to the port and the phone (or access point) powers on. Once the phone is powered on and booted it can negotiate for less power if it wants to and the switch is capable of doing so- otherwise the switch will simply continue to supply full power and the phone will operate normally.

"But wait!" you say. If the transmit and receive pairs are connected wouldn't every transmited packet come right back on the receive pairs and look like a collision? It would- except as we just mentioned- the phone has a Low-Pass filter. It will only pass the low frequency signal from one pair to the other. High frequency signals (like ethernet packets) are blocked.

How 802.3af works, and how to make it work with your Cisco 79xx phone:

An excellent explanation of how 802.3af works is available from PowerDsine.

There are two types of devices- the PSE (Power Sourcing Equpment) and the PD (Powered Device). The PSE can be either Type A or Type B. Type A uses the data pairs to send power (pairs 1,2 and 3,6), Type B uses the spare pairs (4,5 and 7,8). 802.3af DOES NOT SPECIFY A POLARITY FOR THE POWER. Your Type A PSE can choose to make pair 1,2 positive and 3,6 negative OR it can choose to make pair 1,2 negative and 3,6 positive (Two wires are used for each leg to increase the power carrying capability of the cable. Cat5 uses 24 gauge wire and a single strand for each leg would not be capable of delivering enough power for certain devices. Also, because ethernet uses differential signaling, there is no way to use a single wire for this purpose. You can only bias the differential pair- not a single wire.). The same is true for a Type B PSE. Also, 802.3af DOES NOT SPECIFY WHETHER TYPE A OR TYPE B POE IS TO BE USED. The PSE can be either Type A, Type B or both (Just not at the same time!). You have no way of knowing what your PSE is until you test it (or if the spec sheet tells you).

In other words- in order to make your Cisco Pre-Standard phone work with 802.3af you need to know whether your PSE is Type A, Type B, or if it can do both and you need to know the polarity. You can determine the PSE type by putting a 25kohm resistor across the 1,2 - 3,6 pairs (pins 1 and 3) and checking for power, then doing the same for pairs 4,5 - 7,8 (pins 4 and 7). If you get power on 1,2 - 3,6 it is Type A. If you get power when the resistor is across 4,5 - 7,8 then you have a Type B PSE. If you get power either way then your PSE can do both. If your PSE is Type B or it can do both then you are set. Determine which pair is positive and which pair is negative (because 802.3af can go either way). Make an ethernet cable that takes the negative power from the spare pairs and puts it on pair 7,8 (Check this first to be sure but I'm nearly certain this is correct) and the positive on pair 4,5. This might be a straight through ethernet cable, or if the power from your PSE is backwards, you may need to crossover pairs 4,5 and 7,8.

Things get a little more complicated if your PSE is a Type A. I don't know which pairs are positive and which are negative when the data pairs are used. With my NetGear FS108P I was only getting power on the data pairs (it's a Type A PSE). I plugged the phone into my switch with a 25kohm resistor across the data pairs and a straight through ethernet cable but it would not power on. Figuring the polarity might be backwards I made up a standard crossover cable, plugged it in and the phone powered on! (Just like with the Type B PSE you may need a straight through cable or a crossover cable depending on the polarity of your PSE). The best part is that the crossover cable doesn't cause any network problems because the NetGear switch does auto-MDIX on every port. In other words the switch corrects for the crossover cable, but the power polarity remains the same. Translation: the phone works perfectly using 802.3af POE and connects to the network. The complicated bit is that if you need a crossover cable to correct the polarity, and your switch doesn't support auto-MDIX, then you're going to get power but no ethernet signal.

If you have a NetGear FS108P just put a resistor across pins 1 and 3 and connect your phone to the switch using a crossover cable. (I connected my phone with reverse polarity several times without issue but your mileage may vary- I offer no guarantees. Several other people have mentioned applying reverse polarity to their Cisco's with no ill effects).

I am not sure if I should put this here or not, but...
I don't have any sort of POE switch, and thought I should be able to make a midspan POE injector.

I spent quite a bit of time trying to find out how to power my 7940/60 G using POE.
This page has lots of blurb but does not actually tell you what to do, other web pages just say to swap pins 4/5 7/8, but still thin on the ground.
RJ45 POE example
Pin # Description Notes
4-VeNegative 48 volts
5-VeNegative 48 volts
7+VePositive 48 volts
8+VePositive 48 volts

I took a two outlet face plate, drilled two holes and mounted female power connectors, I took a ethernet cable cat it in half and punched down pins 1,2,3,6, this cable goes to your switch.
I connected the female power connector to pins 4/5 and 7/8 and the RJ45 outlet on the faceplate now has ethernet and power.

You probably do this at your own risk, etc! But in the end I just had to give it a go, since I could not work out what type of phine I have and what pins took power etc.
I am not sure if I have 7940 or 7940G phones.

You should read the rest of the page since it give useful info on what not to do as well.

The above added by stinga on 12-May-2008

UPDATE1: I've personally tested Cisco 7940G, 7960G and 7912G with a Linksys SRW224P switch (which only provides 802.3af power on the data pairs) and it does work with a small modification: place a 25K resistor across pins 2 (green - 568A) and 6 (orange - 568A) at the switch side and wait a few seconds for the POE light on the port to turn on, then plug in the phone and it powers up (you may need to increase the power allocation for 7940/7960). You can just leave the resistor in place. Note that you should not attempt to plug in any other device as the switch will fry the PHY transceiver in any other device that is not 802.3af compliant. You can purchase the resistor from Digikey Corporation - - part number 25.5KXBK-ND

UPDATE2: The Cisco 7940G and 7960G function correctly using the above method, but the several 7912G I used did power up but indicated that the Ethernet was disconnected.

Standard 802.3af equipment is classified into Power Sourcing Equipment (PSE) and Powered Devices (PD). There are two ways equipment can be powered an which way to use is decided by the PSE, typically a switch or an injector. Type A supplies power over the data pairs, and thus only occupies four wires for 100BaseT. This is only possible if the PSE is a switch. All Cisco 802.3af and prestandard switches use this method. Type B supplies power over the four spare wires in a 100BaseT connection.

The power supplied over a Type A connection with 802.3a is compatible with all Cisco implementations. The only problem is the discovery protocol used, which will be discussed later. For Type B PSEs, Cisco devices come in two versions, the older devices (7940G and 7960G) are incompatible with 802.3af, and pair 1 and 4 need to be swapped for the device to be powered correctly. Newer Cisco equipment (7905G, 7941G, 7961G) is pin compatible with 802.3af, and no swapping is needed.

Most PoE injectors do not perform discovery, which is a way for the PSE to determine whether the device can be powered over the ethernet connection, and thus supplies power to all devices. For this reason, most newer Cisco devices simply work with injectors, and the 7940 and 7960 work with the wire swapping trick. More advances injectors, and practically all PoE switches implement detection to determine whether a device supports being powered over the ethernet connection. This is where the Cisco equipment differs most from 802.3af equipment. Cisco equipment is fitted with an induction loop between the two ethernet pairs, and a low pass filter, which the switch can use to pass an A/C signal from pair 2 to pair 3 and check if the device can accept power. 802.3af devices are discovered by checking for a 25kOhm resistor between pair 2 and 3. Thus a Cisco phone, eg. a 7940G can be connected to an 802.3af switch by adding a resistor between pin 3 and 5 either in the wall-jack or somewhere along the wire, as this will tell the PSE to supply power to the device. If the PSE is type A, no rewiring is needed.

I don't want ot ruin my 7960G so I haven't done any more extensive testing, but I have a 7940G in the office that I'm going to take apart and test the heck out of. I'll post the results of my testing when I'm done.

For my Cisco 7960/40 and the NetGear FS726TP Smart Switch
with POE related experiences, see below.

The information in the following paragraph only works for certain POE Switches- you need to read the information towards the top on determining POE Type.

It was recently brought up on the IRC channel that the 7940 and 60
phones from Cisco do not support the IEEE 802.3af Power Over Ethernet
(which you have to pay for). At first, I didn't believe it, so I grabbed a 7960 and
tried it with a standard 3COM POE injector and found it to be true.
Only the new 7970G phones will work with the IEEE 802.3af standard.

But hope is not lost! Denon suggested that they would work if you
simply reversed the polarity of 2 of the pairs of wire. He said
that by swapping 4 with 7, and 5 with 8 you could use a standard
POE injector to power the phone.

Since I had a crimp tool and some cable laying around, I gave it
a whirl and it works. If you aren't familiar with making cable,
don't try this at home.

Basically, you just wire one side of the cable as standard 568b
and then reverse 4 and 7, and 5 and 8 on the other side. Then,
simply plug your Cisco into a standard POE injector, like the
3CNJPSE ($23.00) and voila! Your phone works.

I've included color mappings for convenience, and you can
reference the following fine site for pretty pictures:

Damin's POE Cable


1 OrWh
2 Or
3 GrWh
4 Bl
5 WhBl
6 Gr
7 BrWh
8 Br

Cisco Side

1 OrWh
2 Or
3 GrWh
4 BrWh
5 Br
6 Gr
7 Bl
8 WhBl

P.S. This hasn't blown up my 7960 yet, but I'm not an electrical engineer,
therefore, use at your own risk and I am DEFINITELY not responsible if you
flip the wrong pairs and turn your unit into a smoking pile of rubble. If
you are that concerned and/or inexperienced with making cables, look on
Ebay and get a used Cisco POE injector, or Power supply. They are
relatively cheap nowadays.

Revised: 3/17/2004 - GJB

The information in the following paragraph isn't entirely accurate. CDP is used to allow the phone to request more or less power from the switch, but if the switch simply supplies full power it would just work. The real problem is that the 7940 and 7960 phones do not have the 25Kohm resistor needed to activate 802.3af POE, and most POE switches do not transmit the low frequency POE detection signal that the Cisco phones loop back to the switch to enable legacy POE modes.

Follow Up 6/18/2004 - GJB

I have a client that has 24 Cisco 7960 phones that they want powered with a single
multiport POE injector. I had originally intended to use the RedHawk BL-6524 unit with
the modified cables, but I called and spoke with an engineer at RedHawk that provided
some more information on why it wouldn't work the way that I expected. "The legacy
Cisco equipment, aside from the inverted polarity, uses a proprietary POE discovery
protocol called CDP (Cisco Discovery Protocol). Therefore, the RedHawk will not be able to
sense that the Cisco is plugged in and will not supply power to the line. The reason it
works with the 3COM 3CNJPSE bricks is that they are dumb units that supply power no
matter what is plugged in." After that conversation, I re-evaluated the options and found that
PowerDSine has a 24 port injector (Model PD-6024) that supports the Cisco Discovery Protocol
and even markets a cable (Cisco Power Splitter, part PD-PS-401-5/CSCS) that I'm willing to bet
is the same design as what we have above. I'm about to place an order for a 24 of the cables
and the injector from Atacomm, so I'll report back how it works."

- confirmed Cisco 7960G phone working with foundry POE switch using inline power legacy mode with NO mods.
- confirmed I have this working on over 10 phones =). Works on Cisco 7940/7960/7970. - Mitchel

The following paragraphs are simply wrong. The 3com power splitter will only work with Type B PSE's that supply power on the unused data pairs. The 3com part is simply a splitter- it has no way of pulling power off the data lines and splitting it out as that would require magnetics (transformers) that are not present in this part. This cable does not work with an NetGear FS108P, for example. I tried it with several different 7940's and 7960's.

Update: 20th August 2006

Guys can I kill this issue (well as dead as possible anyway).

The 3Com part below works correctly to use a standard 802.3af switch power the 'old style' Cisco 7940 and 7960 phones....I say this with certainty because I've used this solution with both the 3com 4400 and D-Link DES-1526 switches.

Just so people are aware there are TWO seperate issues here one is the connections on the phones are wrong, the second issue is that the phones issue a Cisco Discovery Protocol (CDP) during boot up to try to ascertain whether POE (or more correctly inline power) is available. If the switch does not issue a CDP response (and many won't) then your only solution (that I'm aware of) is to use the 3com part below.
The above statement is just plain wrong. The CDP requests are used to allow the phone to request more or less power on those devices that can negotiate power requirements. In the event there is no response the phone simply moves on. It is perfectly capable of accepting POE power without CDP as the many other people in this thread can attest. Myself included.

The part actually is a splitter, it takes the POE over the normal 8 core cable and splits it into a 48v connection and a data connection. This means the phone doesn't have to issue a CDP as it's getting it's power from the power connection (i.e. It's a Dumb connection)

This is not how CDP is used! From Cisco's own web site: "For Cisco pre-standard PDs, if CDP is enabled on the switch, 15.4W is initially allocated, and then further refined when the CDP message is received from the PD. If CDP is disabled on the switch, or if the PD does not support the Power requirements field of the CDP message, the initial allocation value of 15.4W is used throughout the duration of the connection."

3Com sell a part 3CNJVOIP-CPOD which converts the 802.3af correctly to use with any 802.3af switch. It works by taking the 802.3af feed and splitting it into a date connector and a 48v feed for the phone.

I'll investigate the latest firmware to see if it still issues a CDP (I haven't checked a 'crosswired' cable for quite a while and if Cisco have removed CDP from the later firmwares that could provide a different solution and report back.

The Netgear FSM7326P switch supports the pre-standard PoE mode/detection required for Cisco Phones (7910/40/60). This works with standard ethernet cables and does not require the special cable above.

The PowerDSine Midspan injectors together with the PD-PS-401/Cisco "splitter" also work perfectly.

I've tried a Cisco 7960G phone with STANDARD ethernet cables to a D-Link DES-1526 Smart POE switch. This configuration does NOT seem to work. The link lights on the switch light up for the phone, but the phone does not ever power on. Options within the switch's PoE settings are Enable/Disable and Wattage. Changing these has no effect with this phone. Dlink tech support had no specific experience with this phone, but advised that it probably wouldn't work with this phone. (:frown:)

The cable mod does not work either.

Update 01/27/2006:

I have gotten the 7940G phones to work with the DES-1526! There must be other ways to get them to work, but based on a release note from an old SIP release I plugged our phones into a Cisco 2950 switch and let it get some CDP infomation while booting it from a PW130 inline injector. Afterwards I plugged it into the DES-1526 using one of the mod'ed cables and it recognizes the PoE and boots just fine. I would imagine you might be able to go into the config menu and change the Media Type to auto to force this same behavior.

Additional info

I bought a cheapie PoE switch for another office running a test install and figured that the PowerDsine PD-PS-401/Cisco adapter would set me straight and allow the inexpensive TrendNet TPE-S88 PoE switch power them up. The PowerDsine worked on my Asante PoE switches, but the TrendNet was no go. So I took one of my Polycom IP501s and plugged it in using its Polycom supplied PoE cable (SoundPoint IP LAN/Power Cable IEEE802.3af) and it worked. So i took the Polycom cable apart to try and figure out how it works. Well it is beyond me (damn circuit boards) so while not paying attention, I used the Polycom cable with the Cisco 7960 (40s too) and low and behold it worked. I ordered enough Polycom cables for the remaining Ciscos and now i am golden. i Called TrendNet to tell them about my experience, and I was stopped in mid sentence with "Cisco is not supported" I tried to finish and tell the "support" guy that I got it working, but I guess they don't want any part of the Cisco IP phone market.

Seacrest Out, of the closet.

NetGear FS726TP Smart Switch with POE

Created: 2006-May-26

Revised: 2006-Jun-01

I just received two NetGear FS726TP Smart Switches with POE and quickly connected one to my existing switch. I moved one Cisco 7960 (pre IEEE 802.3af standard) VoIP phone to port 12 on the new POE switch using a standard straight-through CAT5 patch cable. Ports 1 through 12 are POE capable (Max 100 Watts), where 13-24 do not support POE, and 25/26 do not support POE either. The phone did not power up. I checked out the default configuration on the POE switch and learned that the POE capability is disabled by default on the switch (with the V1.0.0_0623_A firmware only). I immediately enabled POE on port 12 where my Cisco 7960 is plugged in and the phone did not power up either!

Upon reading the documentation, the switch follows a procedure outlined in the protocol that allows the POE device to tell the switch what its power needs are (NetGear, 2006). For an understanding of POE click here. The reason it doesn't work seems to be that Cisco guessed wrong with respect to the then emerging IEEE 802.3af POE standard. So, to solve the problem between these newer IEEE 802.3af compliant switches and these pre-IEEE 802.3af compliant devices you either have to make your own cable, or purchase a little nifty box as detailed below.

I just visited and purchased 20 of these POE Passive Splitters made by PowerDSign part number: PD-PS-401/CISCO and you might be able to still find them at if they haven't broken the link. For a list of suppliers.

Updated: I received the POE Passive Splitters, and connected one as per instructions and verified that the POE Switch was configured correct, and NOTHING! I doesn't work. I did more research and found this, which basically says that Cisco 7960 (the original models) will NEVER WORK with 802.3af compliant switches because these older phones use something called "Cisco Inline Power" which isn't similar to POE defined by the 802.3af standard.


NetGear, (2006). FS726TP Smart Switch Product Information. Retrieved May 26, 2006, from NetGear Support Web site: link

NetGear, (2006). What is PoE? (Power over Ethernet). Retrieved May 26, 2006, from NetGear Support Web site: link

New Cable configuration - 8 Jul 2012

Hi all, I wanted to share my success in getting my Cisco 7940/7940G phones to boot off a standard POE switch. I used a TP-Link TL-SF1008P this is unmanaged so there were no options to configure. Using the cable configuration above, all I would get is a ack light flashing on the switch but it would not power up. I thought I would make sure that I had wired the cable correct so did it again. Still nothing, whilst researching I noticed the phone had booted up, I had wired the plug incorrectly with the following configuration
Orange white
Green white
Blue White
Brown White
(same as listed above)
Cisco end
Orange White
Green White
Blue white
Brown Blue
Brown White

Try this, however don't expect the phones to boot straight away. I can't figure out what exactly causes them to boot, switching them to another switch port sometimes does it, sometimes they take 15 minutes to boot. Having the SIP firmware (instead of cisco) seems to improve power up time, but be patient and they should boot. Once they do boot, they work perfectly, including the data port on the back.
Usual disclaimer: this worked for me, if you smell smoke, this is your risk not mine.
If this helps anyone out, I'd be curious to know if it works on other standard POE switches (I assume it would) so please either post here or shoot me an e*mail
Created by: Damin, Last modification: Tue 10 of Jul, 2012 (02:12 UTC) by g00r
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