Supported Protocols And Standards; Powered-Device Detection And Initial Power Allocation - Cisco 3750G - Catalyst Integrated Wireless LAN Controller Configuration Manual

Chapter 11 Configuring Interface Characteristics

Supported Protocols and Standards

The switch uses these protocols and standards to support PoE:
•
•
•

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:
•
•
Table 11-1
Class
0 (class status unknown)
1
OL-8550-02
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, Catalyst 3750 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.
A Cisco pre-standard powered device does not provide its power requirement when the switch
detects it, so the switch allocates 15.4 W as the initial allocation for power budgeting.
The initial power allocation is the maximum amount of power that a powered device requires. The
switch initially allocates this amount of power when it detects and powers the powered device. As
the switch receives CDP messages from the powered device and as the powered device negotiates
power levels with the switch through CDP power-negotiation messages, the initial power allocation
might be adjusted.
The switch classifies the detected IEEE device within a power consumption class. Based on the
available power in the power budget, the switch determines if a port can be powered.
these levels.
IEEE Power Classifications
Maximum Power Level Required from the Switch
15.4 W
4.0 W
Catalyst 3750 Switch Software Configuration Guide
Understanding Interface Types
Table 11-1 lists
11-7