Dell EMC Configuration Guide for the S3100 Series 9.14.2.4

Determine the Power Priority

Dell Networking OS uses a sophisticated port prioritization algorithm to determine which ports should receive power so that the PoE and PoE+ ports are powered up and down deterministically.

Dell Networking OS uses the following four parameters for defining the power priority for a port:

1. Power inline mode: class or static.
2. Power inline priority configuration.
3. LLDP-MED priority sent by the PD in the Extended Power-via-MDI TLV or the priority sent by the PD in the IEEE 802.3at power-via-mdi TLV.
4. Slot and port number.

Dell Networking OS maintains a sorted list of PoE/PoE+ ports based on the above parameters. The static ports have higher weight than class mode ports. Hence all static ports always stay on top of all class ports, regardless of the other parameters. Within the set of static ports, the Dell Networking OS attempts to order the ports based on the second parameter, power inline priority, the default of which is Low. If the Dell Networking OS finds multiple ports with the same power-inline priority, it breaks the tie using the third parameter, the LLDP-MED Priority or power-via-mdi priority the PD advertises, which can be Critical, High, or Low. If the Dell Networking OS still finds a tie, the priority is based on the fourth parameter, which is the position of the port number in the switch. There cannot be a tie based on this parameter.

The Dell Networking OS dynamically sorts this list when:

• The power-inline mode or priority changes.
• The PD advertises a different LLDP-MED priority or power-via-mdi priority
• The PD is connected or disconnected

The Dell Networking OS always uses this sorted list of ports for allocation. When you add an extra power supply unit (PSU), the additional ports are powered based on this list. If you remove a PSU, the same list is used to remove power from the ports with lowest priority.