Comment 12 for bug 1758919

IMHO, this should also be fixed in cloud-init. If the input netplan contains "global" routes, the renderer (or whatever can pre-process the Netplan before renderering) should intelligently determine which interfaces have an on-link gateway that matches the global route, and automatically render the route at interface scope instead of "global".

Arguably, if the route's gateway address doesn't match an on-link prefix, it should not be installed anyway (the kernel will reject it anyway, unless the `onlink` flag is supplied, which instructs the kernel to assume the address is on-link even if it doesn't appear to be). But the only useful scenario I can see for supporting the `onlink` flag is if we're installing a route on an interface that will get is IP address via DHCP.