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Understanding Interface Types
Power over Ethernet (PoE) Ports (WS-C2928-24LT-C only)
PoE is supported only on ports 1-8 of the model WS-C2928-24LT-C switch.
Note
PoE switch ports automatically supply power to these connected devices (if the switch senses that there
is no power on the circuit):
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A powered device can receive redundant power when it is connected only to a PoE switch port and to an
AC power source. After the switch detects a powered device, it determines the device power
requirements and then grants or denies power to the device. The switch can also sense the real-time
power consumption of the device by monitoring and policing the power usage.
This section has this PoE information:
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Supported Protocols and Standards
The switch uses these protocols and standards to support PoE:
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Catalyst 2928 Switch Software Configuration Guide
12-4
Cisco pre-standard powered devices (such as Cisco IP Phones and Cisco Aironet access points)
IEEE 802.3 af-compliant powered devices
Supported Protocols and Standards, page 12-4
Powered-Device Detection and Initial Power Allocation, page 12-5
Power Management Modes, page 12-5
Power Monitoring and Power Policing, page 12-6
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.
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.
Chapter 12
Configuring Interface Characteristics
OL-23389-01
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Troubleshooting
