Rockwell Automation 1783-Mxxx Stratix 8000 and 8300 Ethernet Managed Switches User Manual - Manual (Page 65)

Rockwell Automation Publication 1783-UM003I-EN-P - March 2014 65
Switch Software Features Chapter 3
With PoE+, if you do not manually configure the cutoff-power value, the module
automatically determines it by using the device IEEE classification and LLDP
power negotiation or CDP power negotiation with a Cisco end device. If CDP or
LLDP are not enabled, the default value of 30 W is applied. However, without
CDP or LLDP, the module does not allow devices to consume more than 15.4 W
of power because 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 and 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 module 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 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.
Because the module supports external removable power supplies for PoE/PoE+
and can configure the budget per the power supply used, 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 module does not have enough power for the powered
devices, the module denies power to the PoE ports that are in Auto mode
in descending order of the port numbers. If the module 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 module now has more power available, the module grants power to the
PoE ports in Static mode in ascending order of the port numbers. If it still
has power available, the module then grants power to the PoE ports in
Auto mode in ascending order of the port numbers.
IMPORTANT The total wattage of the power supply must be manually configured via the
Device Manager Web interface or CIP for power to be assigned accurately.