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Catalyst 3750 SwitchSoftware Configuration Guide
OL-8550-09
Chapter12 Configuring Interface Characteristics
Understanding Interface Types
Power over Ethernet Ports
PoE switch ports automatically supply power to these connected devices (if the switch senses that there
is no power on the circuit):
•Cisco pre-standard powered devices (such as Cisco IP Phones and Cisco Aironet access points)
•IEEE 802.3 af-compliant powered devices
A powered device can receive redundant power when it is connected only to a PoE switch port and to an
AC power source. This section has this PoE information:
•Supported Protocols and Standards, page12-7
•Powered-Device Detection and Initial Power Allocation, page12-8
•Power Management Modes, page 12-9

Supported Protocols and Standards

The switch uses these protocols and standards to support PoE:
•CDP with power consumption—The powered device notifies the switch of the amount of power it
is consuming. The switch does not reply to the power-consumption messages. The switch can only
supply power to or remove power from the PoE port.
•Cisco intelligent power management—The powered device and the switch negotiate through
power-negotiation CDP messages for an agreed power-consumption level. The negotiation allows a
high-power Cisco powered device, which consumes more than 7 W, to operate at its highest power
mode. The powered device first boots up in low-power mode, consumes less than 7 W, and
negotiates to obtain enough power to operate in high-power mode. The device changes to
high-power mode only when it receives confirmation from the switch.
High-power devices can operate in low-power mode on switches that do not support
power-negotiation CDP.
Before Cisco IOS Release 12.2(25)SE, PoE-capable switches (without intelligent power
management support) caused high-power powered devices that supported intelligent power
management to operate in low-power mode. Devices in low-power mode are not fully functional.
Cisco intelligent power management is backward-compatible with CDP with power consumption;
the switch responds according to the CDP message that it receives. CDP is not supported on
third-party powered devices; therefore, the switch uses the IEEE classification to determine the
power usage of the device.
•IEEE 802.3af—The major features of this standard are powered-device discovery, power
administration, disconnect detection, and optional powered-device power classification. For more
information, see the standard.