AP gets power through PoE even when port CDP is disabled

[manju.cisco]

Hi All,

When an access point was connected to the switch port, it did draw power of 15 watts even when the port on the switch has cdp disabled on it.

cdp was enabled only in global but was disabled on this switch port.

All these days i was thinking that a device to draw power from switch needs to have cdp enabled( like IP Phones for example).

I would like to know how the AP got 15watts power with cdp disabled on port. The AP was 1242AG and the switch is 3750v2.

Can someone help to explain me how the AP draw power even when cdp is disabled on switch port.

Thanks in advance.

Manjunath

[ankugarg]

Hi Manju,

CDP is just to negotiate the actual power required...POE port will give the maximum power it can give, irrespective of cdp...If you have cdp enabled,it will also negotiate the power required by device according to ieee class of device.

-Ankur

[ankugarg]

Also,POE follows the ieee 802.3af standard or cisco specific standard to provide the power...CDP or LLDP has no role at this point.

[manju.cisco]

Hi Ankur,

Thanks,

But in that case how does a IP Phones doesnot gets registerd when the port had cdp disabled.

When i enable cdp on port IP Phones gets registered.

Am i thinking wrong here with respect to AP and IP Phone ?

[ankugarg]

Hi Manju,

So you are talking 2 entirely different problems here with respect to IP phone and AP.

You were saying that AP shouldn't get powered on when CDP is disabled.But you are also saying that phone got power,but didn't get register when cdp is disabled.

So first point is regarding supplying poe when cdp is disabled.Since phones get power,AP should also get power .It  is independent of cdp as i explained in the first post.

Second point is regarding phone not getting registered.You are trying manual registration or automatic regsitration?.In case of automatic registration,phone should get dhcp offer which has all the necessary info for phone to try registering(contact CUCM etc.)...In case of manual registration,you need ot give mac address and other things of phone manually on CUCM..I am not very sure about it,but it may require some info from cdp or lldp packet.

Regarding registration part,i'm not very sure.Just wrote with whatever little understanding i have with regard to this.

-Ankur

[manju.cisco]

Thanks Ankur,

the IP phone got registered after cdp was enabled thought it was not getting registering when cdp was diabled on switch port.............i really not sure if cdp is the real issue, or i might have misunderstood  :-|

???

[Peter Paluch]

Manju,

Ankur is correct here. When an IP phone is initially connected to a PoE-enabled port, the switch will determine that a device is connected that needs inline power. The Cisco proprietary method is to listen for link pulses on the Rx pair that the switch itself sends on the Tx pair - the phone internally loops back the Tx and Rx pair together while powered off. The IEEE method is to measure the resistance between selected wires and if it is determined to lie within predefined boundaries, the switch will allocate a predefined power for this port along with powering it.

After the device is powered on, it may use the CDP or LLDP to precisely advertise the necessary power to the switch which may allow the switch to make more precise accounting for its total power budget. However, both CDP and LLDP may come into play only after the device is already powered and capable of producing and processing the CDP/LLDP messages - in other words, you cannot rely on receiving CDP/LLDP messages to activate a powered port because the connected device does not produce those packets without being powered in the first place.

You may want to read the following URLs for more information about the PoE powering:

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/troubleshooting/power_over_ethernet.html

http://en.wikipedia.org/wiki/Power_over_ethernet

Regarding Cisco IP phones, they use CDP to negotiate, in particular, the Voice VLAN and the QoS override settings for data received from the PC port. If the CDP is deactivated on the switch, the phone will be unable to determine the Voice VLAN and will instead try to obtain its IP settings in the access VLAN of the port. Usually, the DHCP server in this access VLAN does not provide the address of the TFTP server where the configuration files for the IP phone are accessible, and that is the reason why the IP phone is unable to register successfully - because it is suddenly located in a completely different VLAN with no idea where the TFTP server and the configuration files are to be found.

Best regards,

Peter

[glen.grant]

   To turn off power to the AP on the port you must use the command , "power inline never "  .