Server Management

I have a Virtual Windows Server 2012R2 DHCP server supporting roughly 20 scopes / 1000 clients.

I want to setup a physical secondary server to configure DHCP Fail-over.

1. What happens when a fail-over event occurs, the secondary server takes over DHCP, then the primary comes back online? Does the Primary resume and secondary goes back into standby or does primary stay in standby?

2. I believe I am going with Hot-Standby option but would the load-balancing have a similar outcome in a primary down event?

Thank you!