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.
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 theswitch 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.
switch classifies the detected IEEE device
within a power consumption class. Based on the available power in the power
switch determines if a port can be powered.
Table 1 lists these levels.
Table 1 IEEE Power
Level Required from the
30 W (For IEEE
802.3at Type 2 powered devices)
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). Theswitch performs power-accounting
calculations when a port is granted or denied power to keep the power budget up
After power is applied
to the port, the
switch uses CDP to determine the
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.
With PoE+, powered devices
use IEEE 802.3at and LLDP power with media dependent interface (MDI) type,
length, and value descriptions (TLVs), Power-via-MDI TLVs, for negotiating
power up to 30 W. Cisco pre-standard devices and Cisco IEEE powered devices can
use CDP or the IEEE 802.3at power-via-MDI power negotiation mechanism to
request power levels up to 30 W.
allocation for Class 0, Class 3, and Class 4 powered devices is 15.4 W. When a
device starts up and uses CDP or LLDP to send a request for more than 15.4 W,
it can be allocated up to the maximum of 30 W.
power consumption requirement is referred to as the
consumption requirement in the software configuration guides and command
switch detects a fault caused by an
undervoltage, overvoltage, overtemperature, oscillator-fault, or short-circuit
condition, it turns off power to the port, generates a syslog message, and
updates the power budget and LEDs.
The PoE feature operates the
same whether or not the
switch is a stack member. The power budget
switch and independent of any other
switch in the stack. Election of a new
switch does not affect PoE operation. The
switch keeps track of the PoE status for
switches and ports in the stack and includes
the status in output displays.