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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-25303-03
Chapter 1 Configuring Interface Characteristics
Interface Types
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 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), and Power-via-MDA TLVs, for negotiating power up to 30
W. Cisco prestandard 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.
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.
Note The CDP-specific power consumption requirement is referred to as the actual power consumption
requirement in the Catalyst 3750 and 3560 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 Catalyst 3750-X stackable switch also supports StackPower, which allows the power supplies to
share the load across multiple systems in a stack when you connect the switches with power stack cables.
You can manage the power supplies of up to four stack members as a one large power supply For more
information about StackPower, see Chapter 1, “Configuring Catalyst 3750-X StackPower.”
Table 1-1 IEEE Power Classifications
Class Maximum Power Level Required from the Switch
0 (class status unknown) 15.4 W
14 W
27 W
3 15.4 W
4 30 W (For IEEE 802.3at Type 2 powered devices)
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