Hi folks,
I am struggeling with a rather unexpected problem: I have a Raspberry 3B+, running on Bookworm 64bit, all updates installed. I want to use passive PoE, i.e. these simple adapters/PoE-Injectors that use 4 wires of the Ethernet cable to transmit power, leaving only 4 wires for data, limiting transfer speed to 100 MBps. Like these: https://www.amazon.com/iCreatin-Passive ... B00NRHNPUA
On the switch I can see that the Raspberry PI connects with 100 MBps full duplex, and the MAC is learned into the ARP cache, so layer 2 seems fine. However, the Raspberry fails to acquire an IP, and I cannot connect, ping fails.
If I connect the same Pi to a different network port with Gigabit Ethernet, no PoE-Injector, all is fine.
When I use an USB-to-Ethernet Gigabit adapter, attach it to one of the USB ports of the Pi, on the cable in question, with the PoE-injector in place, all is good, IP is acquired.
When I add to config.txt the lineand attach it to a normal Gigabit Ethernet cable, no PoE-Injector, the behaviour is the faulty one: Link established, MAC learned, but no IP address acquired, no connection possible.
This looks like a strange bug in the LAN78XX-Driver...
Any help?
Thanks,
Caerandir
EDIT: Found a workaround - If I disable autonegotiation on the switch and set the switch to fixed 100 MBps full duplex, things work out. Hope this info helps to fix the bug!
I am struggeling with a rather unexpected problem: I have a Raspberry 3B+, running on Bookworm 64bit, all updates installed. I want to use passive PoE, i.e. these simple adapters/PoE-Injectors that use 4 wires of the Ethernet cable to transmit power, leaving only 4 wires for data, limiting transfer speed to 100 MBps. Like these: https://www.amazon.com/iCreatin-Passive ... B00NRHNPUA
On the switch I can see that the Raspberry PI connects with 100 MBps full duplex, and the MAC is learned into the ARP cache, so layer 2 seems fine. However, the Raspberry fails to acquire an IP, and I cannot connect, ping fails.
If I connect the same Pi to a different network port with Gigabit Ethernet, no PoE-Injector, all is fine.
When I use an USB-to-Ethernet Gigabit adapter, attach it to one of the USB ports of the Pi, on the cable in question, with the PoE-injector in place, all is good, IP is acquired.
When I add to config.txt the line
Code:
dtparam=eth_max_speed=100This looks like a strange bug in the LAN78XX-Driver...
Any help?
Thanks,
Caerandir
EDIT: Found a workaround - If I disable autonegotiation on the switch and set the switch to fixed 100 MBps full duplex, things work out. Hope this info helps to fix the bug!
Statistics: Posted by caerandir — Sun Aug 03, 2025 7:08 pm