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POE Best Practices for EX2300/​EX3400/EX4300

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Article ID: KB31588 KB Last Updated: 22 Oct 2020Version: 3.0
Summary:

This article explains the configurations and best practices to be followed in a Power over Ethernet​ (PoE) enabled environment where a PoE voltage injection is observed.

Platforms that apply: EX2300/​EX3400/EX4300.

Symptoms:

PoE stops working on Juniper EX3400/EX2300/EX4300 devices during initial installation.
It is noticed that the new PDs connected to the POE devices do not power up.

Reboot or performing a Junos upgrade to the latest version does not solve the problem.

Cause:
  1. When the PoE ports of the Switches - EX2200P, EX3200P, EX4200P, are connected to the PoE ports of EX3400P/EX4300P/EX2300P, the PoE functionality would be affected on EX4300/EX3400/EX2300P switches. This is an expected behavior.
     
  2. When an EX3400 is connected to another EX3400 or an EX4300 is connected to an EX3400 the PoE operates in a normal manner - This is an expected behavior.

Debugging and confirming voltage injection by analyzing the PoE port status on Juniper switches

Voltage injection on PoE port scan can be identified by enabling 'syslog any any' on EX3400/EX2300/EX4300.

In show log messages, any POE voltage injection port with status 36 indicates this problem. This is caused due to PoE power injected to the ports on EX3400P/EX4300P/EX2300P.

Root Cause Analysis

RCA of port status 36 is due to the mutual powering of switches. This is because Juniper switches support Legacy device detection. Legacy devices are the PDs manufactured before the IEEE POE standard ratified, which uses Capacitor detection, such as Old Cisco Phones.

Switches that supports legacy detection will detect other switches as a legacy device, and power each other.
 
Over time, Microsemi has improved their software and hardware to deal with this condition. Hence, we see different behavior with each chipset PD63000, PD69000, PD69100, etc.
 
The latest behavior identified was with PD69200.  When this condition occurred, the port with status 36 will go down and will not power on subsequent ports.

Example

Topology:
EX3400P Virtual Chassis --------- EX3200P
 
show log messages | match /0/47
Feb 21 17:06:44   chassisd[3487]: PoE port name ge-0/0/47 status 36 <---Power is Injected from the connected device
Feb 21 17:06:44   chassisd[3487]: PoE port name ge-1/0/47 status 36<
 
show poe interface | grep /0/47
ge-0/0/47    Enabled     OFF     15.4W      Low            0.0W           not-applicable
ge-1/0/47    Enabled     OFF     15.4W      Low            0.0W           not-applicable

Solution:

When connecting uplink devices to EX3400P/EX4300P/EX2300P, make sure PoE is disabled on the interface [uplink device].
POE must be enabled only on interfaces where Access Points/VOIP Phones or any other POE powered devices are connected.

Use the following command

set  poe interface <interface name>

When connecting EX4200P/EX3200P/EX2200P to EX4300/EX3400/EX2300, disable PoE on legacy switches (EX4200P/EX3200P/EX2200P) as below: 

set poe interface ge-0/0/0 disable

Modification History:
2020-10-20: Article reviewed for accuracy; no changes required.

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