@frank-w, what do you mean by only put eth0 up? I don’t have eth1, though I have a redundant net.eth1 service. I’m not sure if that matters. I do use ifconfig up and have other configure on it that previously worked on the official kernel.
Kernel 4.4 have no dsa user ports (wan,lanX) which representing the 5 physical ports. There you work with the cpu-ports directly (eth0=lanX,eth1=wan) which were statically connected to ports in switch.
In kernel 4.14+ this is not possible as switch driver is a dsa driver. Here you set eth0 only up (ip link set eth0 up) and configure lanX/wan
All ports are separated similar to vlan,but routed over eth0 nic