Actual combat case: PoE switch cannot supply power to a terminal abroad? Based on LLDP negotiation

The case shared in this issue is related to wired network issues.

Background
A company in Shenzhen is an automation technology company that mainly provides automation solutions. The company purchased a domestic PoE switch (power supply equipment PSE) and used it with a foreign brand reader (power receiving device PD). It was found that the power supply was often insufficient.

Problem phenomenon
After further analysis, it was found that the PD could not work in PoE+ (802.3at) mode, but only in 802.3af mode. As a result, the maximum power of the PD device could only work at no more than 15w, but the actual power of the device could reach 25w, so there was insufficient power supply.

Further checking the manual of the PD device, it needs to be based on LLDP PoE power negotiation to make itself work in PoE+ mode. The description of the device is as follows:

Troubleshooting analysis
Step 1: Clearly eliminate the direction


Confirm whether the PSE device (domestic PoE switch) can send LLDP messages normally.
Confirm whether the PD device (foreign card reader) can send LLDP packets normally.
Gain a deep understanding of the LLDP PoE negotiation process, the meaning of each field in the packet, and whether it complies with the protocol regulations.
Step 2: Capture the LLDP packet of the PD device for analysis

Use a PC to access the switch, monitor the data packets connected to the PD device port, and capture the LLDP packet for analysis.



It is found that the LLDP message sent by the PD device can be received. The value corresponding to the Class level field is 5, that is, class 4 802.3at, and the power supply standard reference is:

So, from the message field, there is no problem with the PD device. Let's take a look at the PSE, which is the PoE switch.

Step 3: Capture the LLDP packet of the PSE device for analysis

The LLDP packet of the PSE is captured as follows:


Obviously, there is a problem. The Power Class field in the field is defined incorrectly. The protocol defines that the corresponding value of the port supporting Class 4 should be 5, but here it is 4, which means it only supports Class 3, that is, 802.3af mode.

Step 4: Draw a conclusion on the problem

There is a problem with the PoE switch that supports LLDP: the Power Class field is defined incorrectly. The protocol stipulates that the corresponding value of the port supporting Class 4 should be 5, but it is actually wrong. When the switch receives the LLDP packet sent by the powered device that supports the 802.3at standard, it does not reply to the LLDP with the 802.3at standard, resulting in the final negotiation of Class 3, that is, 802.3af mode. Therefore, the problem of insufficient power supply occurs.



Solution
The problem was solved by upgrading the switch software version. The LLDP sent after the upgrade is as follows:

Ensure that the PD powered devices in the network can operate normally in PoE+ mode (802.3at) to meet the power requirements of the devices.

From this case, we can see that there are few examples of LLDP negotiating PoE, but when dealing with network device compatibility issues, it is necessary to have a deep understanding of the device working mechanism and related protocols, and carefully analyze the data packets to accurately find the root cause of the problem and solve it.