Ethernet port on USB hub overrides on-board ethernet

Question

I've got a powered USB hub with an ethernet port connected to my Pi, but I'm only interested on the USB ports, as it already has on-board ethernet. However, for some reason, when the Pi boots, the hub's ethernet appears to take over and the on-board ethernet is disabled. Meaning its got no internet, because the cable is plugged in to the Pi, not the hub. Once I disconnect the hub, the lights on the on-board ethernet light up and it connects to the internet. If I then reconnect the hub, its interface shows up but the on-board remains default.

I've tried turning on predictable interface names in raspi-config and disabling the interface in /etc/rc.local to no avail. The network settings on the Pi are pretty much stock except for a static IP address for eth0 in /etc/dhcpcd.conf.

How can I permanently disable the hub's network interface, and why would it seemingly override the on-board one?

ip -details link (no hub)

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 promiscuity 0 minmtu 0 maxmtu 0 addrgenmode eui64 numtxqueues 1 numrxqueues 1 gso_max_size 65536 gso_max_segs 65535
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT group default qlen 1000
    link/ether dc:a6:32:e7:b9:07 brd ff:ff:ff:ff:ff:ff promiscuity 0 minmtu 68 maxmtu 1500 addrgenmode none numtxqueues 5 numrxqueues 5 gso_max_size 65536 gso_max_segs 65535
3: wlan0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DORMANT group default qlen 1000
    link/ether dc:a6:32:e7:b9:08 brd ff:ff:ff:ff:ff:ff promiscuity 0 minmtu 68 maxmtu 1500 addrgenmode none numtxqueues 1 numrxqueues 1 gso_max_size 65536 gso_max_segs 65535

ip -details link (with hub)

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 promiscuity 0 minmtu 0 maxmtu 0 addrgenmode eui64 numtxqueues 1 numrxqueues 1 gso_max_size 65536 gso_max_segs 65535
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT group default qlen 1000
    link/ether dc:a6:32:e7:b9:07 brd ff:ff:ff:ff:ff:ff promiscuity 0 minmtu 68 maxmtu 1500 addrgenmode none numtxqueues 5 numrxqueues 5 gso_max_size 65536 gso_max_segs 65535
3: wlan0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DORMANT group default qlen 1000
    link/ether dc:a6:32:e7:b9:08 brd ff:ff:ff:ff:ff:ff promiscuity 0 minmtu 68 maxmtu 1500 addrgenmode none numtxqueues 1 numrxqueues 1 gso_max_size 65536 gso_max_segs 65535
4: enx2c16dba05ff7: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast state DOWN mode DEFAULT group default qlen 1000
    link/ether 2c:16:db:a0:5f:f7 brd ff:ff:ff:ff:ff:ff promiscuity 0 minmtu 68 maxmtu 9194 addrgenmode none numtxqueues 1 numrxqueues 1 gso_max_size 16354 gso_max_segs 65535

Note that the on-board ethernet worked when these details were taken as the hub was plugged in after boot.

I've also tried adding denyinterfaces enx2c16dba05ff7 to /etc/dhcpcd.conf as suggested by Ingo, but after rebooting I still have no internet until I unplug the hub.

ip -details link for the hub interface after this change:

3: enx2c16dba05ff7: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ether 2c:16:db:a0:5f:f7 brd ff:ff:ff:ff:ff:ff promiscuity 0 minmtu 68 maxmtu 9194 addrgenmode eui64 numtxqueues 1 numrxqueues 1 gso_max_size 16354 gso_max_segs 65535

Answer 1

As you can see the additional ethernet device on the USB hub is detected with interface enx2c16dba05ff7 on the RasPi. Now it is a routing issue what interface is preferred and preference may vary depending on the order the interfaces are detected. On boot up interface enx2c16dba05ff7 is preferred before eth0, when plugged in later, eth0 is already initialized and preferred.

But we need not to fiddle with the routing table. Because we just do not need the hubs interface, we can simply disable it. I assume you are using default dhcpcd networking. So just add a line to /etc/dhcpcd.conf:

denyinterfaces enx2c16dba05ff7