Simple definition
DHCP failover enables cooperating DHCP servers to maintain IPv4 lease availability if one server becomes unavailable.
Purpose and use
Windows Server DHCP failover maintains IPv4 address allocation between two partner servers, using load balancing or hot standby.
How does it work?
Partners exchange lease state over TCP 647. Client DHCP exchanges remain separate and normally use UDP ports 67 and 68.
Example and verification
Test whether clients in every VLAN can reach both partners, particularly through DHCP relays, and inspect partner synchronization state.
Points to watch
This Microsoft mechanism applies to IPv4 scopes. Scope setting changes require replication to the partner; not every configuration change is synchronized automatically.
In short
Availability also depends on relays, networking and consistent configuration, rather than simply having two running DHCP services.