80/20 Rule

You will probably install more than one DHCP server so that the failure of any individual server will not prevent DHCP clients from starting. However, DHCP does not provide a way for DHCP servers to cooperate in ensuring that assigned addresses are unique. Therefore, you must carefully divide the available address pool among the DHCP servers to prevent duplicate address assignment.

For balancing DHCP server usage, use the 80/20 rule to divide scope addresses between DHCP servers. Figure 4.11 is an example of the 80/20 rule.

Figure 4.11 80/20 Rule Model

DHCP Server 2 is configured to lease most (about 80 percent) of the available addresses. DHCP Server 1 is configured to lease the remaining addresses (about 20 percent).

This scenario allows the local DHCP server (DHCP Server 2) to respond to requests from local DHCP clients most of the time. The remote or backup DHCP server (DHCP Server 1) assigns addresses to clients on the other subnet only when the local server is not available or is out of addresses. This same rule can be used in a multiple-subnet scenario to ensure the availability of a DHCP server when a client requests a lease.