The device 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 device determines the device power requirements based on its type:
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The initial power allocation is the maximum amount of power that a powered device requires. The device initially allocates this amount of power when it detects and powers the powered device. As the device receives LLDP or CDP messages from the powered device and as the powered device negotiates power levels with the device through LLDP or CDP power-negotiation messages, the initial power allocation might be adjusted.
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The device classifies the detected IEEE device within a power consumption class. Based on the available power in the power budget, the
device determines if a port can be powered.
Table 1. IEEE Power Classifications
Class
|
Maximum Power Level Required from the Device
|
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 device monitors and tracks requests for power and grants power only when it is available. The device tracks its power budget (the amount of power available on the device for PoE). Thedevice 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 device uses LLDP or CDP to determine the protocol-specific power consumption requirement of the connected Cisco powered devices,
which is the amount of power to allocate based on the LLDP or CDP messages. The device adjusts the power budget accordingly. This does not apply to third-party PoE devices. The device processes a request and either grants or denies power. If the request is granted, the device updates the power budget. If the request is denied, the device ensures that power to the port is turned off, generates a syslog message, and updates the LEDs. Powered devices can also
negotiate with the device 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 LLDP/CDP or the IEEE 802.3at power-via-MDI power negotiation mechanism to request power levels up to 30 W.
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.
The protocol-specific (LLDP or CDP) power consumption requirement is referred to as the actual power consumption requirement in the software configuration guides and command references.
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If the device 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 device is a stack member. The power budget is per device and independent of any other device in the stack. Election of a new active device does not affect PoE operation. The active device keeps track of the PoE status for all devices and ports in the stack and includes the status in output displays.