I've discovered a bizarre problem where if I connect a laptop directly via ethernet to my OpenWrt router, the connection doesn't work. Wirelessly, the connection is fine. Wired to a different router, the connection is fine.
Offending Laptop: 2009 Macbook Pro 5,5
OpenWrt 19.07.6 device: Newifi D2 MT7621
When plugged. the `logread` from the OpenWrt device looks like this (repeating):
mtk_soc_eth 1e100000.ethernet eth0 port 0 link up
mtk_soc_eth 1e100000.ethernet eth0 port 0 link down
mtk_soc_eth 1e100000.ethernet eth0 port 0 link up
mtk_soc_eth 1e100000.ethernet eth0 port 0 link down
relevant JournalD log entries on the laptop
NetworkManager manager: (enp0s10): new Ethernet device (/org/freedesktop/NetworkManager/Devices/2)
NetworkManager device (enp0s10): state change: unmanaged -> unavailable (reason 'managed', sys-iface-state: 'external')
relevant Kernel Log entries on the laptop
forcedeth 0000|0:0a.0 enp0s10: renamed from eth0|
forcedeth 0000|0:0a.0 enp0s10: MSI enabled|
forcedeth 0000|0:0a.0 enp0s10: no link during initialization|
Now my first thought was "bad cable" or "broken ethernet port". But I've eliminated these. I am fortunate to have enough hardware to test every combination:
2 computers, 2 routers, 2 ethernet cables, 2 ISPs, 1 USB ethernet adapter, 2 operating systems,
5 sheets of high-powered blotter acid.
The suspect laptop connects fine via the Ethernet port to the 2nd router (GL.iNet). It also connects fine to the OpenWRT router using a USB Ethernet dongle. All possible connections are ok except for when I connect the Macbook Pro to the OpenWRT box via the ethernet port. This computer has a dual boot, and the SAME results are encountered both on Linux and on MacOS, so this eliminates even a software or network driver issue. The only common denominator is the physical ethernet port connected to the OpenWRT router.
I've given up trying to figure out what is going on. I've already reflashed the router, etc. Any idea's on next steps to troubleshoot exactly why this connection is not successful?