ISC's DHCP Failover has some positive aspects, however, the overall configuration (from our internal testing) is less than optimal. The positives - especially for large organizations - include:
- tracking of your entire IP Address Pool across all DHCP servers
- a standard network router/switch configuration across your entire enterprise for DHCP helper addresses (assumes a segmented broadcast domain in IP4)
- some very basic load balancing of DHCP traffic
- no more single points of failure for DHCP in your environment
From our testing, though, it takes a fairly long amount of time for the server failover to actually happen and ties up some of your IP resources even after failover has occurred. So, some negatives:
- failover is still something of a manual process (mitigated somewhat in releases greater than 4.2)
- IP Pool is split across two servers
- increased configuration complexity
As far as your scenario (having fewer IPs than potential leases) this wouldn't help, as each system needs to reserve part of the pool for itself, which could lead to IP resource contention.