Cisco Catalyst 3560-X Series, Software Configuration Manual

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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-25303-03
Chapter 1 Configuring Interface Characteristics
Interface Types
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.
• 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. The UPoE feature provides the capability to source
up to 60 W of power (2 x 30 W) over both signal and spare pairs of the RJ-45 Ethernet cable by using
the Layer-2 power negotiation protocols such as CDP or LLDP. An LLDP and CDP request of 30 W
and higher in presence of the 4-wire Power-via-MDI TLV can provide power on the spare pair. For
more information about UPoE, see
Universal Power Over Ethernet, page 1-13 .
Powered-Device Detection and Initial Power Allocation
The switch detects a Cisco prestandard 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 adapter.
After device detection, the switch determines the device power requirements based on its type:
• A Cisco prestandard 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 1-1 lists
these levels.