Allen-Bradley armorstratix 5700, User Manual, Page: 236 / 458

236 Rockwell Automation Publication 1783-UM007I-EN-P - December 2017
Chapter 3 Configure Switch Features
Maximum Power Allocation (Cutoff Power) on a PoE Port
The switch determines the cutoff power on a PoE port in this order.
1.
Manually when you configure the power level to budge for the port
2.
Manually when you configure the power level that limits the power that
is allocated to the port
3.
Automatically when the switch sets the power usage of the device by
using the IEEE classification and LLDP power negotiation or CDP
power negotiation
If you do not manually configure the cutoff-power value, the switch can
automatically determine the value by using CDP power negotiation when
connected to a Cisco end device. If the switch cannot determine the value by
using one of these methods, it uses the default value of 15.4 W.
With PoE+, if you do not manually configure the cutoff-power value, the
switch determines it by using one of the following:
•
The device IEEE classification and LLDP power negotiation
•
CDP power negotiation with a Cisco end device
If CDP or LLDP is 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. Values from 15,400…30,000 mW are allocated based on
only CDP or LLDP requests. If a powered device consumes more than 15.4 W
without CDP or LLDP negotiation, the device can be in violation of the
maximum current limitation. The device can experience a 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.
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 turns on or turns off power on the PoE port. The
maximum power allocation is not the same as the actual power consumption of
the powered device. When you manually set the maximum power allocation,
you must consider the power loss over the cable from the 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 that is 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.