Information about PoE
Power over Ethernet Ports
A PoE-capable switch port automatically supplies power to one of these connected devices if the switch senses
that there is no power on the circuit:
•
a Cisco pre-standard powered device (such as a Cisco IP Phone or a Cisco Aironet Access Point)
•
an IEEE 802.3af-compliant powered device
A powered device can receive redundant power when it is connected to a PoE switch port and to an AC power
source. The device does not receive redundant power when it is only connected to the PoE port.
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-upon 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.
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.
Related Topics
Cisco Universal Power Over Ethernet
Powered-Device Detection and Initial Power Allocation
The switch detects a Cisco pre-standard or an IEEE-compliant powered device when the PoE-capable port is
in the no-shutdown state, PoE is enabled (the default), and the connected device is not being powered by an
AC adaptor.
After device detection, the switch determines the device power requirements based on its type:
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
86
Information about PoE
To Next Page
Loading...
Need help?
General
