poE device discovery on poE port.

Unanswered Question
Dec 19th, 2008

Hi every body!

Let Sw1's port_f0/1) connected to device.

sw1 f0/1-----------------D1

Scenario #1

D1 is poE, sw1 detects the PoE device and supply power.

Now , I replace D1 by non-poE device D2

Would sw1 stop the discovery process once the switch discovered the PoE device? If yes, then sw1 in that case will keep supplying power to non-Poe device D2 and possibly damage the D2(non-poE device).

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2)default power on switch PoE -port:

The default power is 15.4 watt.

That means:

PoE devices connected to poE port are expected to with stand that wattage without being burnt out.

That power is sufficient enough to power the PoE device or to put simply no device requiring more than 15.4 watt is expected on PoE port.

Am i correct?

thanks a lot!

I have this problem too.
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Giuseppe Larosa Sat, 12/20/2008 - 03:56

Hello Sarah,

1)there are recommendations to avoid to swap a POE device with a normal NIC on the fly: it is recommended to unplug the POE device, and to wait 10 seconds before plugging the cable into the non-POE NIC.

For example, if the phone needs to be reset for a change in CCM the port goes down the switch stops to provide poe, then the port is up again and the switch starts again to provide poe

2) according to IEEE 802.3 standard for poe there are some levels but to decide the level a CDP or LLDP communication is needed, if no communication the switch port provides the highest level of power to be sure it is enough for the attached device. So CDP or LLDP can allow to save on the power budget of the device allowing per port poe tuning

Hope to help

Giuseppe

sarahr202 Sat, 12/20/2008 - 05:13

Thanks for your reply GIseppe!

"For example, if the phone needs to be reset for a change in CCM the port goes down the switch stops to provide poe, then the port is up again and the switch starts again to provide poe"

Does switch stop supplying power right away as soon as the port goes down? If yes, then why wait for 10 seconds?

If no, then it makes sense to wait a while before inserting another non-PoE device.

Thanks a lot!

Giuseppe Larosa Sat, 12/20/2008 - 23:13

Hello Sarah,

the timing of power supply are not so fast: the suggestion to wait 10 seconds before plugging in a non POE device is for this reason: the time it takes to reset an IP phone is more then 10 seconds: it has to perform a boot and to read its firmware from flash and to execute it.

I might confused you with this example.

Hope to help

Giuseppe

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