POE with power adapters installed

Answered Question
Mar 25th, 2008

I will be changing our our LAN switches to 3560's with POE. We currently run all our IP phones with power adapters. If I plug an IP phone that has a power adapter into a POE port will it damage the switch or phone or will the switch see that it doesn't need power? I will be going around and disconnecting power on the phones before I plug them into the 3560 but I will most likely miss a couple and I don't want to damage my switch.

Thanks,

Ken

I have this problem too.
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Correct Answer by bmcginn about 8 years 8 months ago

Ken,

The inline power will only be used if it is required. CDP is used by the switch to determine what phone is connected to the port. The switch, through CDP then knows what the power requirement of the phone is. Here is an excerpt of a document:

"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."

The document is located at:

http://www.cisco.com/en/US/products/hw/phones/ps379/products_qanda_item09186a00808996f3.shtml#qa2

The switch, using CDP will see that the phone does not need power. The phone may, for the time it takes for CDP to update, turn off in the transition; but once CDP has updated, the power requirements will work themselves out automatically.

I just tried it here and you'll be fine. See the attachment:

Hope that answers your question!

Regards,

Brad

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Correct Answer
bmcginn Tue, 03/25/2008 - 22:40

Ken,

The inline power will only be used if it is required. CDP is used by the switch to determine what phone is connected to the port. The switch, through CDP then knows what the power requirement of the phone is. Here is an excerpt of a document:

"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."

The document is located at:

http://www.cisco.com/en/US/products/hw/phones/ps379/products_qanda_item09186a00808996f3.shtml#qa2

The switch, using CDP will see that the phone does not need power. The phone may, for the time it takes for CDP to update, turn off in the transition; but once CDP has updated, the power requirements will work themselves out automatically.

I just tried it here and you'll be fine. See the attachment:

Hope that answers your question!

Regards,

Brad

Justin Brenton Tue, 03/25/2008 - 22:44

Hi Ken,

You can disable POE on the switch port that you want to turn off.

To disable POE on a ethernet interface

conf t

int fa0/1

power inline never

Power over Ethernet

The Cisco Catalyst 3560 Series can provide a lower total cost of ownership (TCO) for deployments that incorporate Cisco IP phones, Cisco Aironet® wireless LAN (WLAN) access points, or any IEEE 802.3af-compliant end device. PoE removes the need for wall power to each PoE-enabled device and eliminates the cost for additional electrical cabling that would otherwise be necessary in IP phone and WLAN deployments. The Cisco Catalyst 3560 8-port PoE and 24-port PoE configurations can support 8 and 24 simultaneous full-powered PoE ports at 15.4W for maximum powered-device support. Taking advantage of Cisco Catalyst Intelligent Power Management, the 48-port PoE configurations can deliver the necessary power to support 24 ports at 15.4W, 48 ports at 7.7W, or any combination in between. Maximum power availability for a converged voice and data network is attainable when a Cisco Catalyst 3560 switch is combined with the Cisco RPS 2300 Redundant Power System for transparent protection against internal power supply failures and an uninterruptible power supply (UPS) system to safeguard against power outages.

You can also use this POE calculator to figure out exactly how much power you need and how much you will consume with a given number of IP Phones installed.

Cisco Power Calculator

http://tools.cisco.com/cpc/launch.jsp

HTH, Please rate if so

Regards,

Justin

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