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For the past few years, I've used a pi1 with an 802.3af POE splitter.

The splitter outputs an RJ45 ethernet plug, and micro USB with 5VDC @2.4A.

The splitters RJ45 plug only has 2 pair (4 conductors) connected. From left to right, pins 1, 2, 3 and 6 are connected. My understanding is this limits the connection to 100mbit/sec fast ethernet, but that's OK. I don't need speed, just a reliable connection.

rj45 plug

Recently, I replaced the pi1 with a pi4 (2Gb). If I connect it to the same POE splitter (using the official usb-c to micro USB adapter) then it powers up, but the lights on the pi4 ethernet socket never light up.

Should the pi4 auto-negotiate a 100mbit/sec fast ethernet connection? Or will it only activate the port with a cable that has all 4 pairs connected?

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I cannot give a definitive answer, but I experienced similar problems on a PI3B+ which also has a Gigabit Ethernet interface (even if it can't go beyond 300Mbps).

The basic problem (which IMO is a problem with the Ethernet specs) is that ALL protocol negotiation is performed over the 2 pair common to all speeds.

The end result is that 2 Gigabit capable devices will negotiate 1000Mbps, however unless connected by a 4 pair interface this will fail. If either device is only 100Mbps or 10Mbps capable they should negotiate an agreed lower speed protocol.

There are 2 options; either use a Gigabit Ethernet 4 pair cable OR force the Pi to negotiate a lower speed. There is a setting to force 100Mbps (but for the moment it eludes me) I will post the commands later.

I think the commands are:-

sudo ethtool -s eth0 autoneg off
sudo ethtool -s eth0 speed 100

You should check man ethtool

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    ah! thanks for the pointer. My switch allows each port to be locked to a certain speed, and forcing it to be 100Mbps magically made the POE splitter start working. I'm kind of embarrassed I didn't think of trying that earlier.... – James Healy Oct 2 at 12:08
  • I have heard that these two commands should be merged into one, because autoneg off without the link speed is applied temporarily, and expires after a timeout. – Dmitry Grigoryev Oct 2 at 12:24

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