Cisco 500 series Administration Manual page 198

Managing Power-over-Ethernet Devices
PoE on the Switch
NOTE
CAUTION
Cisco 500 Series Stackable Managed Switch Administration Guide
You can decide the following:
• Maximum power a PSE is allowed to supply to a PD
• During device operation, to change the mode from Class Power Limit to
Port Limit and vice versa. The power values per port that were configured
for the Port Limit mode are retained.
• Maximum port limit allowed as a per-port numerical limit in mW (Port Limit
mode).
• To generate a trap when a PD tries to consume too much and at what
percent of the maximum power this trap is generated.
The PoE-specific hardware automatically detects the PD class and its power limit
according to the class of the device connected to each specific port (Class Limit
mode).
If at any time during the connectivity an attached PD requires more power from the
switch than the configured allocation allows (no matter if the switch is in Class
Limit or Port Limit mode), the switch does the following:
• Maintains the up/down status of the PoE port link
• Turns off power delivery to the PoE port
• Logs the reason for turning off power
• Generates an SNMP trap
When a lower voltage PoE switch is connected to the SG500 series switch with
PoE, and connected via PoE enabled ports on both ends of the connection, the
lower voltage switch lose its ability to power any powered device. To avoid this
condition, either disable PoE support on the SG500 or use a non-PoE port.
Consider the following when connecting switches capable of supplying PoE:
The PoE models of the Sx200, Sx300, and Sx500 series switches are PSE (Power
Sourcing Equipment) that are capable of supplying DC power to attaching PD
(Powered Devices). These devices include VoIP phones, IP cameras, and wireless
access points. The PoE switches can detect and supply power to pre-standard
legacy PoE Powered Devices. Due to the support of legacy PoE, it is possible that
a PoE switch acting as a PSE may mistakenly detect and supply power to an
attaching PSE, including other PoE switches, as a legacy PD.
Even though Sx200/300/500 PoE switches are PSE, and as such should be
powered by AC, they could be powered up as a legacy PD by another PSE due to
false detection. When this happens, the PoE switch may not operate properly and
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