Implementing DHCP failover and load balancing
As shown in the two previous recipes, the installation and configuration of a single on-premises DHCP server is straightforward. However, a single DHCP server represents a single point of failure, which is never a good thing. The solution is always to have a second DHCP server. In earlier versions of Windows, you could do this with two DHCP servers, each with a scope. Typically, you used to split the full set of IP addresses and allow each server to have part of that set. The traditional "wisdom" was to do an 80/20 split – have 80% of the scope supplied by your primary DHCP server and 20% on the backup server.
Independent DHCP servers are an error-prone approach and were never ideal since these independent servers did not coordinate scope details. That 80/20 "rule" was a recommendation for one specific customer scenario (a large firm in the Pacific Northwest) and possibly was not meant to become a best practice...
