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.
detection, the switch determines the device power requirements based on its
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
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.
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
Table 1 lists these levels.
Table 1. IEEE Power
Level Required from the Switch
30 W (For IEEE
802.3at Type 2 powered devices)
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
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
If the 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
is per-switch and independent of any other switch in the stack. Election of a
new active switch does not affect PoE operation. The active switch keeps track
of the PoE status for all switches and ports in the stack and includes the
status in output displays.