I’m using a Raspberry Pi CM4 on the official IO Board, running the latest Raspberry Pi OS. I’ve run into an odd Ethernet issue that I don’t know how to start debugging.
When I connect an Ethernet cable with only 2 twisted pairs (instead of the usual 4), the CM4 usually auto-negotiates down to 100 Mbps, which is the max speed supported by 2-pair cabling. This works consistently when connected to unmanaged switches (e.g., Netgear S350 series).
However, when I connect the same CM4 to a Cisco managed switch (Cisco C1200) with the same 2-pair cable, it fails to negotiate a link at all - no lights, no connection, even after waiting several minutes. This only happens on the Cisco switch.
As a comparison, I plugged a Raspberry Pi 3 into the Cisco switch with the same 2-pair cable, and it successfully negotiated to 100 Mbps.
Questions:
What might be the root cause of this?
Are there known compatibility quirks between the CM4’s Ethernet PHY and Cisco switches?
Where should I start debugging this issue?
When I connect an Ethernet cable with only 2 twisted pairs (instead of the usual 4), the CM4 usually auto-negotiates down to 100 Mbps, which is the max speed supported by 2-pair cabling. This works consistently when connected to unmanaged switches (e.g., Netgear S350 series).
However, when I connect the same CM4 to a Cisco managed switch (Cisco C1200) with the same 2-pair cable, it fails to negotiate a link at all - no lights, no connection, even after waiting several minutes. This only happens on the Cisco switch.
As a comparison, I plugged a Raspberry Pi 3 into the Cisco switch with the same 2-pair cable, and it successfully negotiated to 100 Mbps.
Questions:
What might be the root cause of this?
Are there known compatibility quirks between the CM4’s Ethernet PHY and Cisco switches?
Where should I start debugging this issue?
Statistics: Posted by sbspi — Tue Sep 23, 2025 5:34 pm — Replies 2 — Views 276