7942G and 3550 PoE

Unanswered Question
Dec 11th, 2008

I have a bunch of 7942G that need to connect to access switch 3550 power, the problem is that looks like the 7942G are not getting sufficient power. The phone blinks like is going to boot up but just keeps resetting the power connection and does not come up. However, if I connect the phone to a Cisco 3750 the phone does power up. The documentation list the 7942G as Cisco Inlinepower and 802.3af supported but phone just do not come up.

have you seen this before?

Thanks

Jojo

I have this problem too.
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allan.thomas Thu, 12/11/2008 - 14:33

Hi Jorge, is anything reported within the switch system log? I would expect errors to be reported if there insufficent power to the port.

If you attached a single 7942 to the switch does it boot up successfully?

Are the port configured or set to inline-power auto, this is the default. Each port is capable of 15watts without the necessity for an external PSU so it shouldn't be a problem.

As you said the 7942 support both Cisco inline and 802.3af.

Regards

Allan.

jojo007 Mon, 12/15/2008 - 18:50

Allan

Thanks fo the email

I found out that this scenario will not work as the phone needs 48 DC to work with pre-standard Cisco Inline power, the docuumentation is somehow not clear.

It will work for with the 3550 PWR (Data) but I still need the power brick witch do not make sense. I open a TAC case and basically, they are confirming that I need a power brick or new switches 3560 or 3750.

Thanks for the help.

Jorge

Power

Supports IEEE 802.3af PoE (Class 2). Also supports Cisco Prestandard Power over Ethernet (PoE), allowing powering from any of the Cisco Inline Power-capable blades and boxes, plus Cisco midspan. 48VDC is required; it can be supplied locally at the desktop using an optional AC-to-DC power supply (part number CP-PWR-CUBE-3=) or power injector (CP-PWR-INJ=). Local power options require a corresponding AC country cord (see Table 6).

http://www.cisco.com/en/US/prod/collateral/voicesw/ps6788/phones/ps379/ps8535/product_data_sheet0900aecd8069bb68.html

reidg Tue, 12/16/2008 - 07:28

Hello, i think i'm seeing a similar problem with a 7962G and a 3550 PWR switch, did you get a bug ref with this, the literature certainly says it should work!

jojo007 Wed, 12/17/2008 - 09:54

The literature is somehow no clear to me Cisco TAC provided the below info link

The phone 7942G Reads

Power

Supports IEEE 802.3af PoE (Class 2) (Ok I get it that supports 802.3af.) (Here is where my confusion came aboard.)

Documentation reads.

Also supports Cisco Prestandard Power over Ethernet (PoE), allowing powering from any of the Cisco Inline Power-capable blades and boxes, plus Cisco midspan. (however Cisco is telling me that) 48VDC is still required; it can be supplied locally at the desktop using an optional AC-to-DC power supply (part number CP-PWR-CUBE-3=) or power injector (CP-PWR-INJ=). Local power options require a corresponding AC country cord (see Table 6).

So if a 3550 was to be deploy, we still need the 48VDC to be provided.

I connected the IP Phone to a 3560 and also a 3750 and the phones does come up with no problems.

http://www.cisco.com/en/US/prod/collateral/voicesw/ps6788/phones/ps379/ps8535/product_data_sheet0900aecd8069bb68.html

Still working with Cisco on the issue.

JoJo

jojo007 Thu, 12/18/2008 - 09:37

I have an update

while doing the research I came across this interface command

power inline delay shutdown 20 initial 300

and that apperently did it. Without it the phones just keep rebooting.

After I implemented all my phones work ok ( I have 4 phones connected to the switch.) Before not even one could stay up.

Cisco BU contacted me and told me that this was working on their lab but never told me about the command, oh well

credit goes to

rafaelarvelo

http://www.trixbox.org/forums/vendor-specific-unmoderated/grandstream/unable-use-power-over-ethernet-cisco-3500

Hope this helps someone else

JoJo

rob.huffman Thu, 12/18/2008 - 10:04

Hi JoJo,

Thanks for posting back with your resolution to this issue. +5 points for this good work my friend!

Cheers!

Rob

Trent Hurt Mon, 08/10/2009 - 11:42

Have you had any issues in implementing this command on your switchports? We are facing the same issue with the 7942/7962. Just wondering if this has caused any problems.

Thanks,

Andreas Klein Fri, 12/17/2010 - 09:40

We had the same problem and were sure our switch was failing, we are able to put 7942s on the switch now, thanks!

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