Rockwell Automation 1783-BMxxx Stratix 5700 Ethernet Managed Switches User Manual - Manual (Page 86)

86 Rockwell Automation Publication 1783-UM004E-EN-P - June 2014
Chapter 3 Switch Software Features
Powered Device Detection and Initial Power Allocation
A switch 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 switch determines the device power requirements
based on its type:
• The switch classifies the detected 802.3 af/at compliant IEEE device
within a power consumption class. Based on the available power in the
power budget, the switch 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 switch 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 a PoE+ switch allocates 30 W.
The initial power allocation is the maximum amount of power that a
powered device requires. The switch initially allocates this amount of
power when it detects and powers the powered device. As the switch
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 switch monitors and tracks requests for power and grants power only when
it is available. The switch tracks its power budget, which is the amount of power
available on each PoE port. The switch 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 switch 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 switch updates the power budget. If the request is
denied, the switch 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 5 - 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