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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-21521-01
Chapter 13 Configuring Interface Characteristics
Interface Types
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.
• IEEE 802.3at—The PoE+ standard increases the maximum power that can be drawn by a powered
device from 15.4 W per port to 30 W per port.
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:
• 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. Table 13-1 lists
these levels.
The switch monitors and tracks requests for power and grants power only when it is available. The switch
tracks its power budget (the amount of power available on the switch for PoE). The switch performs
power-accounting calculations when a port is granted or denied power to keep the power budget up to
date.
After power is applied to the port, the switch uses CDP to determine the CDP-specific power
consumption requirement of the connected Cisco powered devices, which is the amount of power to
allocate based on the CDP messages. The switch adjusts the power budget accordingly. This does not
apply to third-party PoE devices. The switch processes a request and either grants or denies power. If the
request is granted, the switch updates the power budget. If the request is denied, the switch ensures that
power to the port is turned off, generates a syslog message, and updates the LEDs. Powered devices can
also negotiate with the switch for more power.
Tab le 13-1 IEEE Power Classifications
Class Maximum Power Level Required from the Switch
0 (class status unknown) 15.4 W
14 W
27 W
315.4 W
430 W (For IEEE 802.3at Type 2 powered devices)