2013 Mac Pro. Using both built-in ethernet ports for 2 separate networks, one a Storage LAN and one an Internet connection. On heavy disk I/O (Disk First Aid, array to array copies, etc) the Storage LAN interface just stops working after a while. No framing errors are logged, the routing table is unaffected (until the arp cache times out), the interface counters increment when I ping, and there are no logged events, but all ARP entries disappear and packets never leave the interface. The Internet interface (which was lightly loaded) is fine. Weirdly, even unplugging and replugging the cable does not reset the interface, and neither does changing IP, resetting the switch port on the other end, or changing flow control. I used to have to reboot to reset the interface, but now I found that switching from 1Gb to 10G and back clears the problem.
I'm dying here, however, because it is not reproducible reliably, and ChronoSync hangs so bad when the shares disappear that I have to force reboot with the power button (force quit or a kill -9 of the parent ChronoSync process doesn't work), so I imagine some underlying mount process hanging is the culprit here.
High Sierra 10.13.3 (17D47)