Cisco IE 4000, Software Configuration Manual

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22
Configuring Interfaces
Understanding Interface Types
Powered-Device Detection and Initial Power Allocation
The switch detects a Cisco pre-standard or an IEEE-compliant powered device when the PoE-capable port is in the
no-shutdown state, PoE is enabled (the default), and the connected device is not being powered by an AC adaptor.
After device detection, the switch determines the device power requirements based on its type:

A Cisco pre-standard powered device does not provide its power requirement when the switch detects it, so the
switch allocates 15.4 W as the initial allocation for power budgeting.
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 switch through CDP
power-negotiation messages, the initial power allocation might be adjusted.

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. Table 1 lists 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.
Power Management Modes
To limit the overall PoE budget of DIN rail switches such as the IE-4000, use the global configuration command power
inline wattage max <4-125> .
Note - This command does not apply to rack-mount switches with integrated power supplies, such as the IE-4010 and
IE-5000.
The switch supports these PoE modes:

auto —The switch automatically detects if the connected device requires power. If the switch discovers a powered
device connected to the port and if the switch has enough power, it grants power, updates the power budget, turns
on power to the port on a first-come, first-served basis, and updates the LEDs. For LED information, see the
hardware installation guide.
If the switch has enough power for all the powered devices, they all come up. If enough power is available for all
powered devices connected to the switch, power is turned on to all devices. If there is not enough available PoE, or
if a device is disconnected and reconnected while other devices are waiting for power, it cannot be determined
which devices are granted or are denied power.
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 (POE+) 30 W (requires LLDP)