To help manage these power loads, vendors of managed switches typically program the switches to provision power one port at a time, this is PoE Port Management. Allowing the switch to power up one port at a time avoids having to provide full power to all ports simultaneously, and potentially running out of power in the process. To read our main article about Power over Ethernet, follow this link.
As an example of how PoE switches are designed to operate, one major vendor of PoE switches provides three power management modes that can be configured on the switch ports:
With this default setting, the switch automatically detects if the connected device requires power. If the switch has enough power, then it grants power to the port, updates the power budget information, and turns on port power on a first-come, first-served basis. If granting power to a given port would exceed the power budget, then the switch denies power, disables power on the port, generates a log message, and updates the port’s LEDs to indicate the status.
With this setting, the switch preallocates power to a port even when no device is connected, and guarantees that power will be available for the port. A port with a static power configuration does not participate in the first-come, first-served model of operation. Because power is preallocated, any device that uses less than or equal to the preallocated amount is guaranteed to be powered when connected to a port configured as static. The statically configured power level is not allowed to be adjusted via power classification or messages carried by the CDP or LLDP protocols.
This configuration disables PoE detection and turns the port into a data-only port.
PoE Monitoring and Power Policing
Vendors can also provide mechanisms for managing ports after they are powered. One major vendor’s implementation includes power monitoring and policing. If a Powered Device attempts to consume more power than the maximum allocated on a port, then the switch can be configured to turn off power on the port, or to just log the event and update the LED status light for the port.
The power policing feature is disabled by default. When this feature is enabled, it uses several pieces of information to determine what the power limit should be. You can configure a maximum wattage level for one or more individual ports (or for all ports on the switch), which becomes the power level at which policing will occur, or you can set an automatic or static power level on each port, which will allocate power dynamically or statically, as you prefer. Finally, you can let the switch port automatically determine the power usage of the Powered Device.
When power policing is configured, the switch polices the power usage sensed at the switch port, which is different than the amount of power that actually arrives at the Powered Device over the cable. The cable distance between the switch port and the Powered Device is responsible for some amount of power loss due to the resistance of the copper cable.
If a device uses more than the maximum power allocated on the port, the policing function on the switch can be configured to take different actions: it can be set to turn off power to the port, or it can be configured to just generate a log message and update the status on the port LEDs.
When policing is enabled, a PoE switch can also be configured to manage the order in which ports are powered. If automatic power policing is configured, then the switch will enable power one port at a time using ascending port numbers: port 1 will be powered first, then port 2, and so on.
It’s possible for a switch to power off ports as well. If a new module is installed in a chassis switch, for example, and that switch now has less power to provide to its PoE ports, then typically the ports will be unpowered in descending order, starting with the highest port number that is powered and working downward until the power budget is sufficient to meet the power required by the remaining ports.
To learn more about Power over Ethernet you should also read these articles:
- Power over Ethernet (PoE) [main article]
- PoE Device Roles and Type Parameters
- How Power over Ethernet Works – PoE Operations
- PoE and Cable Pairs
- PoE Vendor Extensions to the Standard
References and Credits
Ethernet: The Definitive Guide: Designing and Managing Local Area Networks, by Charles E. Spurgeon and Joann Zimmerman (chapter 6)