•
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 10: IEEE Power
Classifications, on page 87 lists these levels.
Table 10: IEEE Power Classifications
Maximum Power Level Required from the SwitchClass
15.4 W0 (class status unknown)
4 W1
7 W2
15.4 W3
30 W (For IEEE 802.3at Type 2 powered devices)4
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.
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.
Note
The CDP-specific power consumption requirement is referred to as the actual power consumption
requirement in the software configuration guides and command references.
Note
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
87
Information about PoE
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