Cisco Catalyst 2928 Software Configuration Manual page 246

Understanding Interface Types
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 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.
We recommend that you enable power policing when PoE is enabled on your switch. For example, if
policing is disabled and you set the cutoff-power value by using the power inline auto max 6300
interface configuration command, the configured maximum power allocation on the PoE port is 6.3 W
(6300 mW). The switch provides power to the connected devices on the port if the device needs up to
6.3 W. If the CDP-power negotiated value or the IEEE classification value exceeds the configured cutoff
value, the switch does not provide power to the connected device. After the switch turns on power to the
PoE port, the switch does not police the real-time power consumption of the device, and the device can
consume more power than the maximum allocated amount, which could adversely affect the switch and
the devices connected to the other PoE ports.
Because the switch supports internal power supplies, the total amount of power available for the powered
devices varies depending on the power supply configuration.
If a power supply is removed and replaced by a new power supply with less power and the switch
•
does not have enough power for the powered devices, the switch denies power to the PoE ports that
are in auto mode in descending order of the port numbers. If the switch still does not have enough
power, it denies power to the PoE ports in static mode in descending order of the port numbers.
• If the new power supply supports more power than the previous one and the switch now has more
power available, the switch grants power to the PoE ports in static mode in ascending order of the
port numbers. If it still has power available, the switch then grants power to the PoE ports in auto
mode in ascending order of the port numbers.
For configuration information, see the
Catalyst 2928 Switch Software Configuration Guide
12-8
Chapter 12
"Configuring Power Policing" section on page
Configuring Interface Characteristics
12-24.
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