12-6
Catalyst 2975 Switch Software Configuration Guide
OL-19720-02
Chapter 12 Configuring Interface Characteristics
Understanding Interface Types
•
The switch classifies the detected IEEE device within a power consumption class. Based on the
available power in the power budget, the switch determines if a port can be powered.
these levels.
The switch monitors and tracks requests for power and grants power only when it is available. The
switch tracks its power budget (the amount of power available on the switch for PoE). 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 the port, the switch uses CDP to determine the
actual
power consumption
requirement of the connected Cisco powered devices, and the switch adjusts the power budget
accordingly. This does not apply to third-party PoE devices. 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 ensures that power to the port is turned off, generates a syslog message, and updates
the LEDs. Powered devices can also negotiate with the switch for more power.
If the switch detects a fault caused by an undervoltage, overvoltage, overtemperature, oscillator-fault,
or short-circuit condition, it turns off power to the port, generates a syslog message, and updates the
power budget and LEDs.
The PoE feature operates the same whether or not the switch is a stack member. The power budget is
per-switch and independent of any other switch in the stack. Election of a new stack master does not
affect PoE operation. The stack master keeps track of PoE status for all switches and ports in the stack
and includes the status in output displays.
Table 12-1
IEEE Power Classifications
Class
Maximum Power Level Required from the Switch
0 (class status unknown)
15.4 W
1
4 W
2
7 W
3
15.4 W
4 (reserved for future use)
Treat as Class 0
