IEEE 802.3 af ,power allocation options on Cisco products

[sarahr202]

Hi everybody.

IEEE 802.3af-2003 describes five (5) power classes that a device may belong to. It should be noted that it is not

mandatory that a PSE vendor implement power classification, as these classifications are optional.

Does Cisco implement this option in its  IEEE- 802.3af -compliant-PoE devices( ip phone.AP etc)?

1)Cisco has taken a conservative approach to allocation of power whereby the PSE will allocate either the IEEE 802.3af class, the negotiated Cisco Discovery Protocol value,or 7W per powered device, depending upon the discovered device.

2)What is the default behavior if we have a cisco 802.af compliant switch and  cisco 802.3 af compliant PoE device i.e will switch allocate power using power classification or uses CDP to allocate requested power or just allocate 7w ?

thanks

[smehrnia]

yes, cisco does provide this feature/classification into its devices.

the default behaviour is to find out the 802.3af class first and then if applicable, exchange CDP to report the more specific maximum peak power. if all fail it then allocates the default 7w.

Hope it Helps,

Soroush.

[sarahr202]

By default Cisco uses the combination of both :

finding the 802.3af class of the attached PoE( optional feature of 802.3af )

CDP to provide to exact power the PoE device requested.

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Thanks and have a great day.

[smehrnia · Accepted solution]

what if the attached device wasn't a Cisco device ?

[Leo Laohoo]

what if the attached device wasn't a Cisco device  ?

I believe the question should be "what if the attached device doesn't support CDP"?

[sarahr202]

if the attached PoE device is not Cisco a device or does not support cdp then it does not provide more granular power allocation. For example, if  switch discovers the class of a PoE device as 3 then 15 watts is subtracted from power budget and 15 watts is provided by the switch. The PoE device might be using less power say 12 watts but the 3 watts unused power will not be returned to power budget. if the attached device were cisco's or it supported cdp, it would have informed the switch about its exact power requirement.

thanks