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:
-
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 lists these levels.
Table 1 IEEE Power
Classifications
Class
|
Maximum Power
Level Required from the
Switch
|
0 (class
status unknown)
|
15.4 W
|
1
|
4 W
|
2
|
7 W
|
3
|
15.4 W
|
4
|
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). Theswitch
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.
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.
![](/c/dam/en/us/td/i/templates/note.gif) Note |
The initial
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.
|
![](/c/dam/en/us/td/i/templates/note.gif) Note |
The CDP-specific
power consumption requirement is referred to as the
actual power
consumption requirement in the software configuration guides and command
references.
|
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.