WeOSPower over Ethernet (PoE)
Introduction
Some products have Ethernet ports with support for Power Over Ethernet (PoE). The purpose of PoE is to transmit power over Ethernet cabling from one device to another. The specification is published as the IEEE 802.3-2018 standard clause 33, where a power supplying device is defined as Power Sourcing Equipment (PSE) and the device receiving the power is defined a Powered Device (PD). The PoE switch products are PSEs, able to supply power to PDs.
Configuration
Configuration of PoE ports is done in the PoE context. An example of how to configure port ethX1:
example:/#> configure example:/config/#> poe example:/config/poe#> port ethX1 example:/config/poe/port-ethX1/#>
Syntax
[no] enable-
Enable/disable PoE on this port. If disabled, no PoE detection and classification will occur and the network port will behave as a non-PoE port. Default: Enabled
- no
- Disable PoE port
[no] priority <low|high|critical>-
Configure PoE allocation priority setting ”priority low” is the lowest priority, while ”priority critical” is the highest. Default: Low
- no
- Resets priority to default ‘low’.
[no] limit <1-30>-
Configure specific PoE Power limit (in Watts) on this port, e.g., ‘limit 20’ to limit the delivered power to 20 Watts. The ‘limit’ setting behaviour depends on the related limit-mode setting. Default: Disabled (no limit)
- no
- Removes port specific power limits.
[no] limit-mode <class|forced>-
The limit setting consists of two modes; class and forced. The ‘limit-mode’ is only applicable for ports with a power ‘limit’ set. Default: Class
- no
- Resets limit-mode to default ‘class’.
show- Shows the port PoE settings.
Status Overview
To get an overview of all port statuses as well as for example the allocated and consumed power, the ‘show poe’ command can be used.
example:/#> show poe PoE System Power Status Maximum Power : 80.0 W Allocated Power : 76 W Consumed Power : 65 W Power Usage : 81 % PORT ENA PRIORITY LIMIT MODE DETECT CLASS POWER ethX1 YES High No Class Good Class3 12 W ethX2 NO High No Class Unknown Unknown 0 W ethX3 YES High No Class Good Class3 13 W ethX4 N/A --- --- --- --- --- --- ethX5 NO High No Class Unknown Unknown 0 W ethX6 YES High No Class Good Class3 12 W ethX7 YES High No Class Open Unknown 0 W ethX8 N/A --- --- --- --- --- --- ethX9 YES High No Class Good Class4 28 W ethX10 NO High No Class Unknown Unknown 0 W ethX11 N/A --- --- --- --- --- --- ethX12 N/A --- --- --- --- --- ---
Products with 160 W total will list the power status for the total as well as the power status for each group.
example:/#> show poe PoE System Power Status Maximum Power : 160.0 W Allocated Power : 46 W Consumed Power : 45 W Power Usage : 28 % Power Group 1 Maximum Power : 80.0 W Allocated Power : 15 W Consumed Power : 15 W Power Usage : 18 % Power Group 2 Maximum Power : 80.0 W Allocated Power : 31 W Consumed Power : 30 W Power Usage : 37 % PORT ENA PRIORITY LIMIT MODE DETECT CLASS POWER PowerGroup ethX1 N/A --- --- --- --- --- --- ethX2 N/A --- --- --- --- --- --- ethX3 YES Low No Class Open Unknown 0 W 1 ethX4 YES Low No Class Good Class3 15 W 1 ethX5 YES Low No Class Open Unknown 0 W 2 ethX6 YES Low No Class Open Unknown 0 W 2 ethX7 YES Low No Class Open Unknown 0 W 2 ethX8 N/A --- --- --- --- --- --- ethX9 YES Low No Class Open Unknown 0 W 1 ethX10 YES Low No Class Open Unknown 0 W 1 ethX11 YES Low No Class Good Class3 15 W 2 ethX12 YES Low No Class Open Unknown 0 W 2 ethX13 YES Low No Class Open Unknown 0 W 2 ethX14 N/A --- --- --- --- --- --- ethX15 N/A --- --- --- --- --- --- ethX16 N/A --- --- --- --- --- --- ethX17 YES Low No Class Open Unknown 0 W 2 ethX18 YES Low No Class Good Class3 15 W 2 ethX19 N/A --- --- --- --- --- --- ethX20 N/A --- --- --- --- --- ---
In order to inspect an individual port’s PoE status, the show poe PORT command
can be used.
example:/#> show poe ethX1
PoE port ethX1
Enabled : YES
Priority : High
Power Limit : No
Detection : Good
Class : Class3
Power Status : OK
Consumed power : 12 W
Current : 245 mA
Voltage : 53477 mV
Power remaining: 3800 mW
In this case the port has detected a class 3 device, and is driving enough current and voltage to power it with 12 W.
PoE Power Classes
When plugging in a PD unit to a PoE port on the switch, the switch will detect the class of the connected PD, depending on the unit’s resistance and thereby its maximum power consumption.
The table below lists the maximum power consumption for PDs of the different PoE classes, as well as the (somewhat higher) power actually allocated by the switch, which considers cable losses. Thus, when admitting a class 0 unit, the switch allocates 15.4 W to ensure 12.94 W reach the PoE unit.
| PoE Class | Max Unit Power Consumption (W) | Allocated Power (W) |
|---|---|---|
| 0 | 12.94 | 15.4 |
| 1 | 3.84 | 4.0 |
| 2 | 6.49 | 7.0 |
| 3 | 12.95 | 15.4 |
| 4 | 25.50 | 30.0 |
It is also possible to configure a maximum power limit on each individual PoE port. When setting a power limit, the default behaviour is that the port is allocated the minimum of the (a) configured power limit, and (b) the power allocated for the attached unit’s class (as listed in the table above). For example, if a port’s power limit is set to 12 W, then 7.0 W is allocated when connecting a class 2 PD, while 12 W is allocated if a class 3 PD is connected. This mode is referred to class based limit mode.
There is an additional limit mode: forced limit mode. The forced mode enables the switch to supply power to non-conforming PDs or even non-PoE devices. This mode goes beyond specification, but can be convenient in certain system setups. The forced mode drives power on the port even when no classification is done, assuming a power ‘limit’ is set for the port.
The following additional classification is made for the connected unit depending on resistance:
-
Good: Ok. A PD is connected (Resistance within specification of PoE class 0-4).
-
Open: Ok. Port not connected. (”Infinite” resistance, i.e., open circuit).
-
Short: Ok, when non-PoE unit is connected (Resistance determined as short circuit).
-
Low: The connected unit is detected as a PD and served, although its the resistance is too low to meet the PoE specification (and too high to be determined as short circuit, i.e. non-PoE unit connected).
-
High: The connected unit is detected as a PD unit and served, although its the resistance is too high to meet the PoE specification (and too low to be determined as unconnected, i.e. open circuit).
Allocation of PoE Power
The maximum power a PoE switch can deliver is limited. When more power is requested than available, the switch will stop/refuse delivering power on the port(s) with lowest priority.
To compensate for limited accuracy in measured power consumption, your PoE switch may allow the measured and allocated power to raise somewhat above the stated maximum power of the product, before power delivery is stopped/refused on some port. The user should still ensure that PoE equipment attached to the PoE switch do not use more than the maximum power of the product in total.
Calculating available power and per-port power limitation
PoE power is allocated to handle max consumption by all admitted devices. For each port, the max consumption is calculated as the minimum value of:
- The power allocated to PDs of the attached class.
- The power limit configured for the port (if set), given that limit mode is class based (default).
The exception is when a power limit is set with limit mode forced; then the maximum allowed consumption is always equal to the configured power limit.
The available power is calculated as max output power of the switch, minus the sum of max power for all (admitted) ports. If a new device is attached, its allocated power will be compared to the available power:
- If there is enough power available to serve the new device, it will be admitted.
- If there is not enough power available to serve the new device, the switch will deliver power to the ports with highest priority. Thus, to admit the new device, one (or more) of the already admitted devices will be declined power.
PoE Port Priority
There are three levels of PoE priority (low, high, critical), which can be configured per port. If there is not enough power to serve all attached PDs, preference will be given to ports with higher priority.
In 160 W capable PoE products there exists two power groups, each group capable of driving 80 W. The PoE port priority is handled within each group along the same principles as with products with one power group.
As situations can occur where the switch must chose between two ports of the same level of configured priority, there is a need for a second level ”tie-break” priority. This tie-break priority is allocated in ascending port order, giving the lowest PoE port the highest tie-break priority. The ”tie-break” priority is as follows (starting with the highest tie-break priority):
- Lynx-3510 PoE Switch: eth3(1), eth4(2), eth5(3), eth6(4), eth7(5), eth8(6), eth9(7), eth10(8)
- Viper-x12A PoE Switch: ethX1(1), ethX2(2), ethX3(3), ethX5(4), ethX6(5), ethX7(6), ethX9(7), ethX10(8)
- Viper-x20A PoE Switch: ethX5(1), ethX6(2), ethX7(3), ethX11(4), ethX12(5), ethX13(6), ethX17(7), ethX18(8)
- 20-port 160 W capable PoE units: Power group 1: ethX3(1), ethX4(2), ethX9(3), ethX10(4) Power group 2: ethX5(1), ethX6(2), ethX7(3), ethX11(4), ethX12(5), ethX13(6), ethX17(7), ethX18(8)