Rockwell Automation 1783-Mxxx Stratix 8000 and 8300 Ethernet Managed Switches User Manual - Manual (Page 62)

62 Rockwell Automation Publication 1783-UM003I-EN-P - March 2014
Chapter 3 Switch Software Features
Powered Device Detection and Initial Power Allocation
A PoE expansion module detects a powered device when a port with PoE
capability is active, PoE is enabled (the default), and the connected device is not
being powered by another power source.
After device detection, the module determines the device power requirements
based on its type:
• The module classifies the detected IEEE device within a power
consumption class. Based on the available power in the power budget, the
module determines if a PoE port can be powered. The table below lists
these levels.
• A Cisco pre-standard powered device does not provide its power
requirement when the module detects it. A port that is not configured for
PoE+ allocates 15.4 W as the initial allocation for power budgeting. A
port that is configured for PoE+ switch allocates 30 W.
The initial power allocation is the maximum amount of power that a
powered device requires. The module initially allocates this amount of
power when it detects and powers the powered device. As the module
receives CDP messages from the powered device and as the powered
device negotiates power levels with the module through CDP
power-negotiation messages, the initial power allocation can be adjusted.
The module monitors and tracks requests for power and grants power only when
it is available. The module tracks its power budget, which is the amount of power
available on each PoE port. The module 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 a PoE port, the module uses CDP (if CDP is supported
by the powered Cisco end device) to determine the actual power consumption
requirement of the connected powered devices and adjusts the power budget
accordingly. The switch processes a request and either grants or denies power. If
the request is granted, the module updates the power budget. If the request is
denied, the module verifies that power to the port is turned off, generates a syslog
message, and updates the status indicators. Powered devices can also negotiate
with the module for more power.
Table 3 - IEEE Power Classifications
Class Power Supplied per Port, max
0 (class status unknown) 15.4 W
1 4 W
2 7 W
3 15.4 W
4 30 W PoE+ devices only