Cisco switch,attached PoE and detection methods.

[sarahr202]

Hi everybody.

There are two ways a cisco switch can detect connected PoE:

By  supplying voltage across Transmit and receive pins, if 25 ohms resistance is detected, switch concludes  802.af compliant PoE device is connected.

The second  method is used by cisco inline method.  A switch transmits a low frequency signals and if the switch gets this signal on its receive pin, switch concludes cisco inline compatible PoE device is connected.

My question is which method switch tries first to detect attached PoE device by default? Because when first switch powers up, it has no clue as to what type of PoE device is connected i.e  if it is 802.1compatiable device or Prestandard Cisco inline PoE device.

thanks and have a great day.

[handoko wiyanto]

hi,

who knows this link might help:

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps5023/QA_Enhanced_Power_over_Ethernet.html

Q. How does Cisco Enhanced PoE negotiate power requirements with powered devices (PDs)?

A. Switches supporting Cisco Enhanced PoE use Cisco Discovery Protocol  (CDP) to autonegotiate a power level in excess of 15.4W per port. The  Cisco Enhanced PoE switch informs the powered device of the power level  it is capable of providing, and the powered device then selects the  appropriate power level.

regards,

[handoko wiyanto]

well,

or this one might do:

http://www.cisco.com/warp/public/cc/so/neso/bbssp/poeie_wp.pdf

[Leo Laohoo]

My question is which method switch tries first to detect attached PoE device by default? Because when first switch powers up, it has no clue as to what type of PoE device is connected i.e  if it is 802.1compatiable device or Prestandard Cisco inline PoE device.

Here's my take ...

Cover your eyes and choose a powered-device and plug to a Cisco switch with PoE.  The power is off to the PD.  How will YOU and/or the Cisco switch know if the PD is a PoE or not?  What power does it require?  Are you 18?  Etc.

So when a PD is plugged into a Cisco PoE switch, the switchport will power up to 15.4w (ok, ok, ok, some of you will jump-up-and-down and correct me that it's 14.0 w).  The power of 14.0w/15.4w will allow the PD to power up.  If the PD will talk Cisco CDP protocol then the PD will send a CDP up the wire in the words like, "Hoy!  You.  Yes you.  I don't need this much power.  All I need is watts."

And correct power is negotiated.

Currently, there are three "sets" of PoE standards in the market:

1.  Standard PoE;

2.  PoE plus (sometimes known as enhanced PoE); and

3.  Cisco's proprietary Universal PoE (aka uPoE).

[sarahr202]

thanks Leolaohoo

But before switch can provide power to the powered device, it must decide if the attached device is PoE or not otherwise it might fry the attached device if it is not a powered device.

So we are back to same question .  Switch must determine if the attached device is indeed powered device, once this determination is made, power  should be provided.  ; we have two detection methods which method does switch uses first to detect powered device?

So when a PD is plugged into a Cisco PoE switch, the switchport will  power up to 15.4w (ok, ok, ok, some of you will jump-up-and-down and  correct me that it's 14.0 w).

But according to the link:

http://www.cisco.com/warp/public/cc/so/neso/bbssp/poeie_wp.pdf

"Cisco Catalyst switches, by default, allocate 7W per port that in dense telephone environments may be inefficient in

terms of the number of powered devices that can be powered."

My understanding is 7 watts is the default value which is enough power for a powered device to power up and request exact power required using cdp. For example if we have an access point attached to switch's port , it will power up using 7 watts but with limited functionality i.e  if it has two antenna( transmitter) one could be off until the access point gets the required power from switch's port.

The value 7 watts is chosen to provide backward compatibilty with prestandard poE devices. For example If the attached poe is cisco ip phone that require max 7 watts to operate., will be fried if the switch uses 15 watts as default value.

Please correct me if iam wrong.

The question i have is:  which detection method cisco uses to detect attached PoE i.e is it detection method suggested by 802.3af standard or switch uses detection method used by cisco inline method.?

===================================================================

Lets get back to the time when  802.af standard did not come out. Cisco used inline method to provide power over ethermet to ip phone which at that time required 6-7 watts to operate. My question is :

Did such PoE still comunicate their power requirement using cdp or  they did not communicate any power requirement using cdp because they already had required power?

thanks and have a great day.

[Leo Laohoo · Accepted solution]

But before switch can provide power to the powered device, it must decide if the attached device is PoE or not otherwise it might fry the attached device if it is not a powered device.

My take is based on my experience deploying wireless access points.  How can a switch determine if the attached device is PoE or not?  So the switch pumps power into, by default.  But the question is:  How much?  I know that the WAPs I deploy won't support 7.0 w and I know that when I do "sh power inline" command the output is "15.0w" and once the WAPs have finished exchanging CDP with the switch it goes to the appropriate level:  15.0w or 20.0w.

[sarahr202]

But according to the link( http://www.cisco.com/warp/public/cc/so/neso/bbssp/poeie_wp.pdf )  switch can still detect if the attached device without pumpimg power into it.

Cisco prestandard device such as ip phone  has low pass filter . Sw sends the frequency tone and if it receives it on receive pin with little attenuation, switch concludes powered device is present.

A cisco 802.3af standard switch can use a different method to detect powered device.  All 802.3af compatible device connect 25 ohms resistance to indicate their existence to 802.3 af compatible switch. So switch  supply a voltage across its transmit and receiver pin and measures the current flowing. If V/I =25 ohms, switch concludes the attached device is 802.3af compatible powered device.

But the question is:  How much?  I know that the WAPs I deploy won't  support 7.0 w and I know that when I do "sh power inline" command the  output is "15.0w" and once the WAPs have finished exchanging CDP with  the switch it goes to the appropriate level:  15.0w or 20.0w.

It could well be possible either wap indicates to switch its power class assuming your wireless access points is 802.af compatible and implements optional power classes as suggested by 802.3 af standard. Or it might be possible wap does not implements any power classes. In that case switch defaults to use 15 watts ( class 0). once it get cdp messages from wap it could adjust it accordingly.

My understanding is just by looking power supplied on a port by a switch we can not conclude the switch always supply 15 watts by default because it could be switch is providing that 15 watts because it could not determine the class of 802.3af PD or PD supports that particular class. What would happen if such port is connected to prestandard phone? If switch uses 15 watts as default value, it will fry that ip phone or worse if the attached device is not PD.

My understaning is switch must determine if the attached device is powered device before it could supply power to it or risk frying it.

thanks

[handoko wiyanto · Accepted solution]

hi all,

i found another one here:

http://www.cisco.com/en/US/products/hw/phones/ps379/products_qanda_item09186a00808996f3.shtml

Q.    How do I determine if the IP phones that receive inline power use the   Cisco pre-standard PoE version or the IEEE 802.3af   standard?

A. Cisco Standards based Power over Ethernet supplies power to Cisco IP       phones, Cisco Wireless access points and any third party IEEE 802.3af standard       compliance powered devices (PDs) with the same Ethernet cable that carries the       data. Cisco Catalyst 3750 and 3560 Power over Ethernet supports both the Cisco       pre-standard Power over Ethernet implementation as well as the IEEE 802.3af       Power over Ethernet implementation. This ensures backward and forward       compatibility and investment protection.

The Catalyst 3750/3560 PoE switches support both the Cisco pre-standard       Power over Ethernet, and the Standards based Power over Ethernet methods of PD       detection. Both detection methods are active at the same time, and either one       can be used in order to detect a valid PD. The Catalyst 3750/3560 PoE switches       periodically check all ports, powered and non-powered, in order to check their       status and the power status of connected devices.

Cisco Catalyst 3750/3560 PoE switches support Cisco pre-standard PD       detection mechanisms, and any Standards based compliant PDs. Most Cisco made       PDs, pre-standard or standard, support Cisco Discovery Protocol (CDP). Once       power is applied to a port that contains a pre-standard or standard Cisco PD,       CDP is used in order to determine the actual power requirement, and the system       power budget is adjusted accordingly.

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.

For Standards based compliant PDs, the Catalyst 3750/3560 Power over       Ethernet controller classifies the PD at the detection stage and allocates a       required power budget based on the IEEE class. If a PD supports both IEEE       802.3af and Cisco pre-standard, the PD is detected as an IEEE device. The       Catalyst 3750/3560 PoE switches classify the PD at the detection stage and       allocate a required power budget based on the IEEE class. Then, a CDP message       determines the actual power usage for the PD, provided that CDP is enabled on       the switch. If the requested power through CDP is higher than the PoE       controller classified power, the requested power is adjusted to the PoE       controller IEEE class.

Since all of this happens automatically, it is not possible to       determine whether the IEEE standard or the pre-standard runs.

regards,

[sarahr202]

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       th...

I quote from the white paper i refered in my very first post.

Since the launch of Cisco Inline Power in March 2000, Cisco

has shipped more than 16 million inline power capable ports on the Cisco Catalyst

3500, 4500 and 6500 series of Ethernet

switches

With the announcement of IEEE

802.3af-2003 compliant 10/100/

1000BASE-T support for Cisco Catalyst

multiservice switches, Cisco delivers

standards based Power over Ethernet that is

backward compatible with the installed

base of Cisco prestandard Power over

Ethernet capable switches.

A Cisco IEEE+CDP powered device, such as a Cisco IP phone 7970G, will come up in low power mode (6.3W) and

will transmit a Cisco Discovery Protocol message with an inline power (ILP) type length value (TLV) that informs

the PSE of the actual power required by the device. If the power is less than the default 15.4W, the PSE will

acknowledge the request with it’s available power and modify the PSEs power budget. If the requesting powered

device exceeds the power budget for the line card or switch, the port will be either powered down, or the port will

remain in low power mode (7W).

This management scheme is implemented to provide backward compatibility and investment protection to the

installed base of Cisco Catalyst prestandard Power over Ethernet capable line cards and switches. Cisco IP phones are power efficient and require 6.3W maximum power as reflected within the prestandard Power over Ethernet

implementation. However, the development of new high-power powered devices, such as wireless access points and

IP phones with color LCD screens require additional power that cannot be delivered using the prestandard

implementations. By bringing up Cisco powered devices in low power mode, Cisco high power powered devices can

operate, albeit with reduced functionality,2 on prestandard line cards.

Follwing can be infered from above:

1)  Prestandard line card on cisco switch can not provide more than 7 watts.

2) Wireless access points and ip phone with color lcd screens require more than 7 watts  if these devices used with prestandard cisco switches, they will operate with reduced functionality.

============================================================

If cisco switch has to be truly compatible with prestandard PoE devices which are mostly ip phone, it must not provide more than 7 watts by default so those prestandard PoE can operate without getting fried.

thanks and have a great weekend.

[sarahr202]

Hi Leolahoo.

I just want to say you are right.   I am surprised as to why there is so many cisco links  with conflicting description for a same topic.

For example:

By contrast, the IEEE 802.3af-2003 standard uses a different powered device detection technique that uses DC detection to determine whether a powered device is attached and to which power classification the device belongs.An IEEE 802.3af-2003 PSE achieves this by applying a DCvoltage between the transmit and receive pairs and measuring either the received current (Amps) or voltage (V) received. A PSE will expect to see a 25K Ohm resistance between the pairs for the device to be considered a valid powered device. If the PSE does not detect a valid 25KOhm resistor, power is not applied to the port.

Once a powered device has been discovered, an IEEE 802.3af PSE may optionally perform powered deviceclassification by applying a DC voltage and current to the port. If the powered device supports optional powerclassification it will apply a load to the line to indicate to the PSE the classification the device requires by attenuating the DC voltage.

In order for PD to connect the load across the line to indicate its power class, it must be powered on which means PD requires power.  It has to happen even before PSE can determines the PD's class.

I was looking a debug produced by " debug il power " and " debug cdp packets"

What I noticed really corroborated your observation:

I know that when I do "sh power inline" command the output is "15.0w"  and once the WAPs have finished exchanging CDP with the switch it goes  to the appropriate level:  15.0w or 20.0w.

Debug outputs indicates following:

802.3 af compatible cisco switch provides the default power of 15.4 watts regardless of PoE connected i.e it could be old prestandard ip phone or it could PD. ( Whether a prestanard ip phone requiring only 7 watts max, could handle 15.4 watts safely before informing switch about its actual power requirement  still boggles my mind)

Once the PD is powered up, it will connect the required load ( which is simply a resistor) across transmission line.

PSE then is able to determine Power class of PD ( assuming PD does implement 802.3 af's optional power class features)

After few exchanges of cdp messages, PSE will either adjust its power on the port.

Now compare this with CIsco white paper which omits this details:

Once a powered device has been discovered, an IEEE 802.3af PSE may optionally perform powered deviceclassification by applying a DC voltage and current to the port. If the powered device supports optional powerclassification it will apply a load to the line to indicate to the PSE the classification the device requires by attenuating the DC voltage. The PSE then determines the powered device’s classification using the Volt-Amp (VA) slope returned by the powered device’s signature. If the powered device does not support classification, the powered device is assigned to Class 0, the default class.

It did not mention at all as to how PD is able to provide load across  transmission line without being turned on.

The white paper also erroneously mentions that  once PSE detects the presence of PD, the next step is to determine the power class of PD or uses the default class 0. 

it is in total contrast withdebug output of " debug ip power" which corroborates your observation that switch by default supplies 15.4 watts. right after PD is detected. It was only once the PD is powered up and connects the load across the transmission line, that PSE is able to determines the class.

Here is what I learned :

Prestandard line cards or switches can provide upto 7 watts.

Cisco switches compatible with 802..3 AF, provides 15.4 watts by default whether the PoE is 802.3 af compatible or some prestandard device. This power  will adjust after few cdp messages exchanges between switch and PoE device.

Thanks and have a great weekend.

[Leo Laohoo · Accepted solution]

I am surprised as to why there is so many cisco links  with conflicting description for a same topic.

The people who were hired to keep Cisco's library of documentations updated are long gone.  The only way is to submit a "Feedback" and request for the documentation to be updated or corrected.

I just want to say you are right. 

I doubt if I am, hence I started with "This is my take ... " as I am no electrical enginner.  I base my knowledge purely on hands-on work.

My question or "tease" to you is this:  Aside from debug, how do you troubleshoot PoE issues?  Try this scenario:

You have a Cisco PD (12.0 watts) and but wouldn't power.  The command "sh power inline" shows the power to negotiate to 15.4w and the description of "IEEE".

Plug this phone to another port on the floor and it powers up.  Put this back to the original port and it won't power up.  Where is the problem?  How do you troubleshoot this?

[sarahr202]

You have a Cisco PD (12.0 watts) and but wouldn't power.  The command  "sh power inline" shows the power to negotiate to 15.4w and the  description of "IEEE".

I get two peice of info from above:

First the switch lets call this switch sw1, supports 802.3 AF standard,

Second, Sw1 is providing default power of 15.4 watts.

Plug this phone to another port on the floor and it powers up.  Put this  back to the original port and it won't power up.  Where is the  problem?  How do you troubleshoot this?

Well. the problem obviously lies with first switch i.e sw1

But I am still at loss why the PD requiring 12 watts does not power up when connected to sw1 providing 15.4 watts by default? The PD has more than enough  power available to power up but it does not.

thanks and have a great weekend.

[Leo Laohoo · Accepted solution]

Well. the problem obviously lies with first switch i.e sw1
But I am still at loss why the PD requiring 12 watts does not power up when connected to sw1 providing 15.4 watts by default? The PD has more than enough  power available to power up but it does not.

I told you I was going to give you something to think about over the weekend!

Have you considered the CABLE?  Out there in the "wild", there are three "sets" of cables.  They are:

1.  Local end (switchport to the patch panel);

2.  Horizontal cabling (self explanatory); and

3.  Remote end

There was a time in the past that evertime one opens a new bag of cable you expect 100%.  You also expect that if you hired a cabler to run horizontal cabling throughout your office, you'd get 100% success rate.  Unforatunately, it's no longer the case.

If you want to learn more about PoE, kindly delve into the ways to deal with what I've described as this is now becoming a "norm", i.  e.  faulty cabling will prevent the PD from powering up and the command "sh power inline" will show 15.4w of power being pushed and under "Device" the output of "Ieee" is shown.   Just like the output below:

Gi2/37    auto   on         16.4       15.4       Ieee PD             3 
Gi9/1     auto   on         16.4       15.4       Ieee PD             3 
Gi9/17    auto   off        16.4       15.4       Ieee PD             3

The devices plugged into the three ports of a 4500R+E are Cisco wireless access points.  Are they working?  Definitely not.  So why, you might want to ask, are they not working?  Is the switch faulty?

There are other WAPs in the same chassis, in the same line card and they power up fine.  So that eliminates that you have a chassis problem.  It also eliminate that you have a line card problem.  This narrows down to two things:  The WAP is faulty or you have a cabling problem.

I can tell you that the WAPs are not faulty because we tested it and swapped new WAPs.  This leaves cable problem.  So where is the problem?  How do you test it?

[sarahr202]

The IEEE 802.3af standard states that power may be delivered by an end-point PSE, using either the active data wires

of an Ethernet port or the spare wires, to a powered device

Sometimes cabler won't bother connecting spare wires at both ends . I will check the cable with cable tester to see if there is no open and  all the wires on each end have continuity

thanks

[Leo Laohoo · Accepted solution]

 I will check the cable with cable tester to see if there is no open and  all the wires on each end have continuity

Cheap cable tester will tell you whether or not you've got a fault and, in most cases, will only test two pairs (out of four).  You will need specialized cable testing gear to tell you HOW FAR is the fault from the cable tester.

However, if you have a Cisco switch, this process will make things alot easier.  The process is called TDR (Time Domain Reflectometer).