Great question — answering as the UphillTech team:
No magic or anything "clever" is happening. When netplan via networkd creates the vlan interfaces, it adds routes to the route table. If you run ip route you see by default the route table main. This table stores the "main" route for the networks, in your case 100.64.8.0/24 100.64.10.0/24 192.168.100.0/24 **Note unless you have set one interface to have a different metric for routes, when the network cables are plugged in, you will not be able to access any ip address on these subnets, because the kernel can not work out what interface to use. if you run ip route show table local You will see the local, broadcast and if set other types of routes that are stored in the local route table. In...
Full answer by Luke Attard: https://askubuntu.stackexchange.com/a/1569285 (CC BY-SA 4.0).
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