Comment 1 for bug 1877643

Would love to see it become a warning for bionic rather than revert.

The previous (lack of) validation allowed invalid configs to silently be accepted, leading to unexpected behaviour when processed by the backend (e.g. networkd).

In our case, we had machines lose connectivity on reboot after a long period of up-time because we had listed the bonding mode number (4) instead of the valid name (802.3ad). IIRC, at the time, we eventually found that this made the bonding mode fall back to round-robin.