During my testing I set the lease time to 42 seconds. I did several "reboot now" on the EdgeRouter X as well. During a reboot cycle of the ERX the client had to renew the lease due to the timeout setting. The client got a DHCP reply from the uplink for ERX!

How can that be? Is the ERX configured to act as a switch on all ethN ports until the bootup scripts configure the ERX as described in /config/config.boot? If so, how can I prevent that?

Imagine what would happen if the ERX has the usual "long" lease time. If it gets rebooted and a client happens to renew the least while the ERX is still in the middle of booting, I think the client will have broken/unusual network settings. And these bad settings remain until either the client is rebooted, or its network is restarted, or the lease expires.

Does that make sense?