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Ask Dr. Cisco

Q. Does the Catalyst 3750 switch support Cisco pre-standard compliant IP phones?

A. The Catalyst 3750 switch supports both the Cisco pre-standard PoE method and the IEEE 802.3af PoE standard. The switches automatically supply power to connected pre-standard powered devices, such as Cisco IP phones and Cisco Aironet access points, and to IEEE 802.3af-compliant powered devices if the switch senses that there is no power on the circuit.
However, there can be issues when you connect some third party pre-standard compliant devices to the Catalyst 3750, because the third party devices can potentially use different pins in order to detect power. When you deal with third party pre-standard compliant devices, check with the manufacturer about the alignment of the pins for power detection.

Q. Can my Catalyst 3560 PoE 48 port switch successfully power 48 Gigabit Cisco Phones?

A. It is not possible to power all 48 phones as the gigabit sets are Class 3 which typically draw between 12.9W and 15.4 W. The total power available is 370W, and for 48 ports, this evenly divides up to ~7.71W per port. In this case, the 3560 switch can only support 28 phones that draw 12.9W each.

Q. I have a Cisco CP-7942G IP phone. I need to add a DSS. Can this be done with this set?

A. No. None of the CP-794x sets can handle any type of add-on module (DSS). You will need to upgrade to a CP-7960G, 7961G, 7962G, 7965G, 7970G, 7971G-GE, or 7975G. The Comparable set to your current CP-7942G would be the CP-7962G. The appropriate DSS for the 7962G is the CP-7915. You can add up to (2) of these units to the 7962G. You'll also possibly need some CP-PWR-CUBE3's. If you are powering the phone with a power cube presently, the 1st add-on module will NOT need any additional power but the 2nd one will. If the phone is being powered via PoE, both the 1st and the 2nd add-on modules will require a local power supply.

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