Requires= on sys-subsystem-net-devices-bond.lan254.device propagates the
device unit's stop, and nothing pulls a service back in when the interface
returns, so a single recreation of bond.lan254 (netdev change, bond flap)
would have ended DHCP and router advertisements silently. Order against the
device with Wants=/After= instead. The ExecStartPre address probe is the real
gate: it waits up to 30s per attempt, so Restart= retries stay well below the
start limit and recover on their own once the addresses appear.

Deployed to xlab-gateway (generation 28); the requisites diff was exactly the
two unit files, system-units, etc and the system itself.

Co-Authored-By: Claude Opus 5 <[email protected]>