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Cisco IP Phone 7970 low brightness level with third party switches



I would like to share some good information why those phones normally has issues with 3rd party vendor switches.

 After some research I was able to find that this phone mode 7970G comes up in a low power mode (6.3W) and transmits CDP in a inline power type length value that informs the Power Source Equipment about what power this device is going to need.

In the other hand IEEE has created the standard POE 802.3at POE also called (POE Plus) which goal was to have the option to increase from 15W to up to 25.5W.

Going back to this 7970G phone we state that  LLDP-MED TLV is supported by this model and in order to support negotiation with other 3rd party switches not supporting the standard Cisco POE (802.3af)  the phone would need to support IEEE 802.3 at or (POE Plus) and the Advance Management LLDP-MED; Unfortunately this model does not support it and as results it boots up its default power (802.3af) and the result is that the phone will not support the full range of brightness options off of this much power.


POE Requirements for some 79XX Phones

  • CP-7902G (6.3W)

  • CP-7905G (6.3W)

  • CP-7910-SW (6.3W)

  • CP-7910G (6.3W)

  • CP-7912G (6.3W)

  • CP-7940G (6.3W)

  • CP-7960G (6.3W)

  • CP-7906G (5W) (Class 2)

  • CP-7911G (5W) (Class 2)

  • CP-7941G (6.3W) (Class 2)

  • CP-7941G-GE (12.9W) (Class 3)

  • CP-7961G (6.3W) (Class 2)

  • CP-7961G-GE (12.9W) (Class 3)

  • CP-7970G (10.25W) (Class 3)

  • CP-7971-G-GE (15.4W) (Class 3)

  • CP-7985G (12.55W) (Class 0, Not full brightness)

  • IEEE 802.3af Device - Class 0 (15.4W)

  • IEEE 802.3af Device - Class 1 (4W)

  • IEEE 802.3af Device - Class 2 (7W)

  • IEEE 802.3af Device - Class 3 (15.4W)

Some good links:

Power over Ethernet (PoE) Power Requirements FAQ


LLDP-MED and Cisco Discovery Protocol


Cisco Universal Power Over Ethernet



Gerson Fabian Morales Marin (gersomor)


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