802.11n LWAP 'draws low power' ??

Unanswered Question
Sep 5th, 2008

Dear all,

we deployed the first 802.11n LWAPs. In some areas where the have anumber of clients associated we get the following alarm:

"Access point 'AP001d.70fa.6128' associated with controller 'Cisco_df:f8:e3' draws low power from Ethernet. Failure reason: 'The AP draws 15.4 watts from Ethernet'"

It seems as if the AP know two higher power levels (up to 20 watts?) whilst the switch only goes to 15.4 watts. Sometimes clients then get disassocieted from this AP. How could I fix this? Is it a firmware upgarde on the switch? Does the PoE switch HW need to be replaced? Or is it harmless and the disassociateions have probably an other reason?


I have this problem too.
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jeff.kish Fri, 09/05/2008 - 07:06

The Cisco 1250, as with all 802.11n access points, requires somewhere in the vicinity of 18W in order to operate. Standard 802.3af PoE only supplies up to 15.4W, so your switch is not able to supply full power to the AP. That gives you two options:

Your first option is to disable one of the radios on your 1250 (either 2.4GHz or 5GHz), and it will operate fine with standard power.

Your second option is to supply full power to this AP. You can do this with a Cisco power injector for the 1250, or you can simply plug it into the wall where it's mounted, if possible. Finally, you can do this through Cisco Enhanced PoE, which requires an E-series switch and a software upgrade.

Once the IEEE releases the 802.3at PoE+ standard, Cisco will make switches that can supply enough power by default. Until then, we're stuck with these limited options.

Scott Fella Fri, 09/05/2008 - 17:03

Jeff is right, but the E-Series switches can give you 20w of power to the ports which will allow you to run the 1252 in full 'N' mode.

lburke Sun, 09/07/2008 - 13:56

If you have an e-series poe switch, then the command to enable the port to allow up to 20watts is the following

int gi1/0/blah

power inline port maximum 20000

Also make sure it's running the latest IOS code.


Anonymous (not verified) Wed, 10/15/2008 - 09:40

Anonymous (not verified) Wed, 10/15/2008 - 09:59

Anonymous (not verified) Wed, 10/15/2008 - 09:52

Anonymous (not verified) Wed, 10/15/2008 - 09:55

Anonymous (not verified) Wed, 10/15/2008 - 10:22

jcerero77 Wed, 09/23/2009 - 08:07

I have WLC code and have switch 3750E version 12.2(35)se5. I have access points 1252's but it seems that my switch is not providing enough power, the max is 15.4 watts. But I thought that this specific switch is capable of doing the max of 20. Do I need an upgrade or a specific command? Any help is appreciated. Thanks.

Leo Laohoo Wed, 09/23/2009 - 15:23

1. Your 3750E is running the wrong IOS. I believe it should be running the 12.4(44) IOS.

2. Next you have to configure extra power to the port using the command power inline port maximum 20000 .

If you don't want to upgrade the IOS of the switch, then upgrade the firmware of the WLC to version 6.X and this will allow the 1250 to operate both radios BUT AT A COST of lower MCS rates.

Hope this helps.

Please rate when this post is helpful or answers your query.

jcerero77 Thu, 09/24/2009 - 08:11

Thanks for your response, I hope you meant 12.2(44) because I couldn't find any 12.4(44) releases. I will upgrade my switches and hopefully this resolves the issue.

dimensyssrl Thu, 01/22/2009 - 07:56


We have a 4507R-E switch with same problem.

But from configuration I can only set 15,4 W maximum...

Is it 4500 limit?



jeff.kish Thu, 01/22/2009 - 13:10

Hi Daniele,

That's correct. The 4507R-E switch cannot provide enough power to support the 1252s. This is a limit of the switch. To power the APs, you'll need to use power injectors or plug the device into the wall. Or consider purchasing a 3560E or 3750E, but that's the most expensive option for sure.

Stephen Rodriguez Thu, 01/22/2009 - 13:33

Information on the 4500 supporting the 1252 is below:

4500 modules req switch to run 12.2(44)SG or later



power inline port maximum 20000

power inline auto max



with_joerg Fri, 01/23/2009 - 02:43


that is certainly true. But, what we observe is t hat if there is a higher WLAN client denisty (6 to 12 clients) on a 1252 AP powered with 15.4W only we get messages in the WCS saying "AP XY draws low power" and clients are disconnected arbitrary. So, it works, yes, but does not play nicely in all cases.

jeff.kish Mon, 01/26/2009 - 07:34


The doc you link states the following:

16.8 W-Both transmitters are used but at reduced power. Legacy data rates are not affected, but the M0 to M15 data rates are reduced. Throughput should be minimally impacted because all data rates are still enabled. The range is affected because of the lower transmit power. All receivers remain enabled.

15.4 W-Only a single transmitter is enabled. Legacy data rates and M0 to M7 rates are minimally affected. M8 to M15 rates are disabled because they require both transmitters. Throughput is better than the legacy rate but less than the 20 and 16.8 W power modes.

So even this document states that only a single radio can be enabled when on 15.4W. Am I missing anything?

Leo Laohoo Wed, 09/23/2009 - 22:02

HI Daniele,

Check my post.

If you don't want to upgrade the IOS of the switch, then upgrade the firmware of the WLC to version 6.X and this will allow the 1250 to operate both radios BUT AT A COST of lower MCS rates.

If you are running autonomous IOS, upgrade the IOS to 12.4(10b)JDA3 and this will 1250 to operate both radios (at 15.4 watts) BUT AT A COST of lower MCS rates.

Hope this helps.


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