DHCP failover is a mechanism whereby two DHCP servers are both configured to manage the same pool of addresses, so that they can share the load of assigning leases for that pool and provide backup for each other in case of network outages.
Failover partnerships are configured per pool, so one DHCP server may have several active partnerships with different peers. The configuration elements of DHCP failover are to establish peering relationship first (via the failover peer declarations in
dhcpd.conf), and then to establish which pools will use the partnership by adding the failover peer option to the pool declarations individually.
For more information and a sample configuration, please refer to: A Basic Guide to Configuring DHCP Failover.