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Maximum Power Allocation (Cutoff Power) on a PoE Port

When power policing is enabled, the switch determines one of the these values as the cutoff power on the

PoE port in this order:

Manually when you set the user-defined power level that the switch budgets for the port by using the

power inline consumption default wattage global or interface configuration command

Manually when you set the user-defined power level that limits the power allowed on the port by using

the power inline auto max max-wattage or the power inline static max max-wattage interface

configuration command

Automatically when the switch sets the power usage of the device by using CDP power negotiation or by

the IEEE classification and LLDP power negotiation.

Use the first or second method in the previous list to manually configure the cutoff-power value by entering

the power inline consumption default wattage or the power inline [auto | static max] max-wattage command.

If you do not manually configure the cutoff-power value, the switch automatically determines it by using CDP

power negotiation or the device IEEE classification and LLDP power negotiation. If CDP or LLDP are not

enabled, the default value of 30 W is applied. However without CDP or LLDP, the switch does not allow

devices to consume more than 15.4 W of power because values from 15400 to 30000 mW are only allocated

based on CDP or LLDP requests. If a powered device consumes more than 15.4 W without CDP or LLDP

negotiation, the device might be in violation of the maximum current (Imax) limitation and might experience

an Icut fault for drawing more current than the maximum. The port remains in the fault state for a time before

attempting to power on again. If the port continuously draws more than 15.4 W, the cycle repeats.

When a powered device connected to a PoE+ port restarts and sends a CDP or LLDP packet with a power

TLV, the switch locks to the power-negotiation protocol of that first packet and does not respond to power

requests from the other protocol. For example, if the switch is locked to CDP, it does not provide power

to devices that send LLDP requests. If CDP is disabled after the switch has locked on it, the switch does

not respond to LLDP power requests and can no longer power on any accessories. In this case, you should

restart the powered device.

Note

Power Consumption Values

You can configure the initial power allocation and the maximum power allocation on a port. However, these

values are only the configured values that determine when the switch should turn on or turn off power on the

PoE port. The maximum power allocation is not the same as the actual power consumption of the powered

device. The actual cutoff power value that the switch uses for power policing is not equal to the configured

power value.

When power policing is enabled, the switch polices the power usage at the switch port, which is greater than

the power consumption of the device. When you are manually set the maximum power allocation, you must

consider the power loss over the cable from the switch port to the powered device. The cutoff power is the

sum of the rated power consumption of the powered device and the worst-case power loss over the cable.

The actual amount of power consumed by a powered device on a PoE port is the cutoff-power value plus a

calibration factor of 500 mW (0.5 W). The actual cutoff value is approximate and varies from the configured

value by a percentage of the configured value. For example, if the configured cutoff power is 12 W, the actual

cutoff-value is 11.4 W, which is 0.05% less than the configured value.

Catalyst 2960-X Switch Interface and Hardware Component Configuration Guide, Cisco IOS Release 15.0(2)EX

80 OL-29034-01

Configuring PoE

Power over Ethernet Ports

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