Rate and give feedback:  Feedback Received. Thank You!

Rate and give feedback: 

X

This document helped resolve my issue

Yes No

Additional Comments

800 characters remaining

May we contact you if necessary?

Name:  

E-mail: 

Enter a valid Email ID

Need product assistance? Contact Juniper Support

Submit

This site is protected by hCaptcha and its Privacy Policy and Terms of Service apply.

English  

English

Understanding PoE on EX Series Switches

Power over Ethernet (PoE) enables electric power, along with data, to be passed over a copper Ethernet LAN cable. Powered devices—such as VoIP telephones, wireless access points, video cameras, and point-of-sale devices—that support PoE can receive power safely from the same access ports that are used to connect personal computers to the network. This reduces the amount of wiring in a network, and also eliminates the need to position a powered device near an AC power outlet, making network design more flexible and efficient.

Note: We recommend that you do not connect an enabled PoE port on one switch to an enabled PoE port on a second switch. If there is a large voltage difference between the power supplies of the two switches, the resulting negative current will trigger a fail-safe mechanism that prevents the power sourcing equipment (PSE) from delivering power to the other PoE ports on that switch.

This topic describes PoE on Juniper Networks EX Series Ethernet Switches.

PoE, PoE+, and Enhanced PoE

PoE was first defined in the IEEE 802.3af standard. In this standard, the amount of power that can be supplied to a powered device is limited to 15.4 W. A later standard, IEEE 802.3at, defined PoE+, which increases the amount of power to 30 W. The PoE+ standard provides support for legacy PoE devices—an IEEE 802.3af powered device can operate normally when connected to IEEE 802.3at (PoE+) power sourcing equipment.

Starting with Juniper Networks Junos operating system (Junos OS) Release 11.1, Juniper Networks provides enhanced PoE on EX3200 and EX4200 switches. Enhanced PoE is a Juniper Networks extension to the IEEE 802.3af standard that provides power of up to 18.6 W per PoE port.

Table 1 lists EX Series switches and line cards and the version of PoE they support.

Note: This topic and its related topics use the term PoE as a generic term for PoE, PoE+, and enhanced PoE.

PoE Power Allocation

A switch or line card that supports PoE has a PoE controller that keeps track of the PoE power consumption on the switch or line card, and allocates power to the PoE ports. The following factors determine how the PoE controller allocates power to the PoE ports:

• Maximum PoE Power Consumption
• PoE Power Budget
• PoE Interface Power Allocation
• PoE Interface Power Priority

Maximum PoE Power Consumption

The maximum PoE power consumption is the total amount of power available for the PoE controller to allocate to all of the PoE interfaces. In allocating power, the PoE controller cannot exceed the maximum PoE power consumption.

How the maximum PoE power consumption is determined depends on the switch model:

• Maximum PoE Power Consumption on EX2200, EX3200, EX3300, EX4200, and EX4300
• Maximum PoE Power Consumption on EX6200 and EX8200 Switches

Maximum PoE Power Consumption on EX2200, EX3200, EX3300, EX4200, and EX4300

The maximum PoE power consumption on EX2200, EX3200, EX3300, EX4200, and EX4300 switches depends on the switch model and the capacities of the power supplies installed. To find the maximum PoE power consumption for each switch model, see Table 2 for EX2200 switch models, Table 3 for EX3200 switch models, Table 4 for EX3300 switch models, Table 5 for EX4200 switch models, and Table 6 for EX4300 switch models.

The maximum PoE power consumption for a switch is displayed in the Maximum power field in the output of the show poe controller CLI command.

• If you change your existing power supply to a lower-capacity power supply, the maximum PoE power consumption might no longer be sufficient to power all the PoE ports on the switch.
• If your switch supports redundant power supplies and you have installed power supplies of different capacities, the maximum PoE power consumption is based on the wattage of the lowest-capacity power supply.
• You cannot increase the number of PoE-capable ports on a switch by installing a power supply that has a higher capacity.

Table 2 lists the EX2200 switch models, number of PoE-enabled ports, power supply ratings, and maximum PoE power consumption.

Table 3 lists the EX3200 switch models, number of PoE-enabled ports, power supply ratings, and maximum PoE power consumption.

Table 4 lists the EX3300 switch models, number of PoE-enabled ports, power supply ratings, and maximum PoE power consumption.

Table 5 lists the EX4200 switch models, number of PoE-enabled ports, power supply ratings, and maximum PoE power consumption.

Table 6 lists the EX4300 switch models, number of PoE-enabled ports, power supply ratings, and maximum PoE power consumption.

Maximum PoE Power Consumption on EX6200 and EX8200 Switches

For EX6200 and EX8200 switches, each line card that supports PoE has its own PoE controller and maximum PoE power consumption. The maximum PoE power consumption is allocated to the line card by the switch’s power management, while PoE power is allocated to the ports on the line card by the PoE controller. Because EX6200 and EX8200 switches can differ in the number and capacity of power supplies installed and in the number and types of line cards installed, the amount of power available for PoE power can vary for switches of the same model.

Power management allocates PoE power to line cards that support PoE only after it has allocated base power to and powered on all line cards. It then allocates the remaining power to the line cards for PoE in order of line card power priority. (In a default configuration, power priority is determined by the line card slot number, with slot 0 having the highest priority.) If the remaining power is insufficient to provide PoE power to all PoE line cards, a low-priority line card might receive no PoE power or partial PoE power.

By default, power management allocates enough PoE power to a line card to power all PoE ports at their maximum supported power. If the powered devices connected to that line card require less power than that, you can configure a smaller maximum PoE power consumption for the line card. For example, power management normally allocates 915 W of PoE power to a 48-port PoE+ 20 Gbps (EX8200-48PL) line card. If the powered devices connected to that line card consume no more than a total of 250 W, you can set the maximum PoE power consumption for the line card to 250 W. Doing so frees 665 W, which then can be used to fulfill the PoE power needs of lower-priority line cards.

You can also configure the power priority of the PoE ports on a line card. If power management is unable to allocate enough power to a line card to meet its maximum PoE power consumption, the line card’s PoE controller turns off power to PoE ports in reverse priority order as required to meet the reduced power allocation.

Power management adjusts PoE power allocations as power availability and demand in a switch change. As a general rule, power management allocates power to power on line cards before it allocates PoE power. For example, if you add a line card and there is insufficient power available to power it on, power management reduces the PoE power it provides to line cards, starting with the lowest priority line card, until it frees enough power to power on the new line card. When power management reduces the maximum PoE power consumption for a line card because of insufficient power, it logs a message in the system log.

Note that the actual power consumed by the powered devices does not affect power management’s power allocation for a line card. If you have set the maximum PoE power consumption for a line card to 500 W, power management allocates 500 W even if the powered devices are consuming less power than that. Similarly, the maximum PoE power consumption is not increased if you add additional powered devices—if the powered devices require more than the 500 W maximum that you have configured, lower-priority devices do not receive power.

You can display the switch’s power budget maintained by power management, including its PoE power allocations, by using the show chassis power-budget-statistics command. You can also display the maximum PoE power consumption for each line card in a switch by using the show poe controller command.

For more information about how power management allocates power, including PoE power, see Understanding Power Management on EX Series Switches.

PoE Power Budget

The PoE power budget is the total amount of power that the PoE controller has available to allocate to its PoE ports. The PoE controller cannot exceed its PoE power budget and does not allocate power to a PoE port if the allocation would exceed the PoE power budget.

How the PoE power budget is determined depends on the class of the powered device:

• The power budget calculation for class 0 and class 4 devices is based on actual power consumption. These values are displayed in the Power consumption field in the output of the show poe interface CLI command. The power budget is the sum of the power consumption values for all PoE-enabled interfaces.

  Note: For Junos OS Releases 12.2R1 through 12.2R5 and 12.3R1 though 12.3R4, the PoE power budget for class 4 devices is based on maximum power.

• The power budget calculation for class 1, class 2 and class 3 devices is based on maximum power. The maximum power for an interface is displayed in the Max power field in the output of the show poe interface CLI command. The power budget is the sum of the maximum power values for all PoE-enabled interfaces.

PoE Interface Power Allocation

The maximum power for a PoE interface is the maximum amount of power that can be provided by that interface. If the actual power consumption of a powered device connected to a PoE interface exceeds the maximum power allocated to that interface, the switch turns off power to the interface.

The maximum power for a PoE interface is allocated by the PoE controller. One of the three methods listed below is used, on a per interface basis, to determine maximum power. The methods are listed in order of priority.

• LLDP Power Negotiation
• Class PoE Management Mode
• Static PoE Management Mode

LLDP Power Negotiation

Link Layer Discovery Protocol (LLDP) power negotiation enables the PoE controller to dynamically allocate power to LLDP-enabled powered devices based on their power needs. The PoE controller allocates to an interface only the power currently required by the connected powered device, and it can allocate the power in small increments.

When the management option for PoE is set to class and LLDP is enabled (both are default settings), LLDP power negotiation is enabled by default. If you disable LLDP power negotiation or the powered device does not support it, the switch uses the class of the powered device to determine the maximum power for interfaces.

Note: LLDP power negotiation is not supported on EX3200 and EX4200 (except EX4200 PX models) switches.

Class PoE Management Mode

In the class PoE management mode, the maximum power for an interface is determined by the class of the connected powered device. The PoE standards IEEE 802.3af and IEEE 802.3at define classes of powered devices based on the levels of power that they require. Table 7 lists the classes of powered devices and associated power levels.

Because of line loss, the power range of the powered device is less than the maximum power delivered at the PoE port for each class. Line loss is influenced by cable length, cable quality, and other factors and is typically less than 16 percent of the maximum power.

The powered device communicates to the PoE controller which class it belongs to when it is connected. The PoE controller then allocates to the interface the maximum power required by the class (see Table 7). It does not allocate power to an interface until a powered device is connected. Class 0 is the default class for powered devices that do not provide class information. Class 4 powered devices are supported by PoE ports that support only IEEE 802.3at (PoE+).

By default, when the management option is set to class and LLDP is enabled, LLDP power negotiation is also enabled on supported switches. See LLDP Power Negotiation for more information.

Static PoE Management Mode

In the static PoE management mode, you specify the maximum power for each PoE interface. The PoE controller then allocates this amount of power to the interface from the maximum PoE power consumption for the switch or line card. For example, if you specify a maximum value of 8.0 W for ge-0/0/3, the PoE controller allocates 8.0 W for this interface out of the maximum power consumption. This amount is allocated to the interface irrespective of whether a powered device is connected to the interface or the connected powered device uses less power than 8.0 W.

Because of line loss, the power received by the powered device can be less than the power available at the PoE port. Table 8 shows the maximum power available at a PoE port and the resulting power guaranteed to the powered device.

PoE Interface Power Priority

You can configure a PoE interface to have a power priority. The power priority determines which interfaces receive power if PoE power demands are greater than the maximum PoE power consumption. If the total power allocated for all interfaces exceeds the maximum PoE power consumption, PoE power to lower-priority interfaces is turned off and the power allocated to those interfaces drops to 0. Thus you must set interfaces that connect to critical powered devices, such as security cameras and emergency phones, to high priority.

Among PoE interfaces that have the same assigned priority, power priority is determined by the port number, with lower-numbered ports having higher priority.

For EX6200 and EX8200 switches, interface power priority determines the relative priority of the interfaces on a line card, not on the switch as a whole. The relative priority of interfaces residing on different line cards is determined by line card priority. For example, if line card 1 has a higher power priority than line card 2 and a power shortage occurs, power is removed from the PoE interfaces in this order:

• Low-priority interfaces on line card 2
• High-priority interfaces on line card 2
• Low-priority interfaces on line card 1
• High-priority interfaces on line card 1

You can manually configure PoE interface power priority, or you can enable LLDP power priority, which assigns each interface the power priority provided by the connected LLDP-enabled powered device. Table 9 describes how the switch converts LLDP power priorities to switch power priorities.

Note: LLDP power priority requires LLDP power negotiation to be enabled, which is enabled by default when the PoE management option is set to class.

Note: LLDP power priority is not supported on EX3200 and EX4200 (except EX4200 PX model) switches.