Using Split-Scope Configurations
Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2
You can increase fault tolerance by splitting DHCP scopes between multiple DHCP servers. With a split-scope configuration, if one server becomes unavailable, the other server can take its place and continue to lease new IP addresses or renew existing clients. Splitting DHCP scopes also helps to balance server loads.
When splitting the IP address pool of a scope between two servers, assign the same scope to both servers, and exclude opposite portions of the address range. You also need to make identical reservations at both DHCP servers, so that either server can assign the reserved IP address, ensuring that the intended device receives the address that is reserved for its use.
Figure 2.4 shows a network that is using a split-scope configuration.
Figure 2.4 Split-Scope Configuration
In Figure 2.4, DHCP Server 1 has 80 percent of the addresses in the scope and DHCP Server 2 has 20 percent of the addresses in the scope. Splitting a scope between servers in this way, which is commonly referred to as the "80/20 rule," often relies on the proximity of the DHCP servers to the clients it serves. For example, when a DHCP client that is on the same subnet as DHCP Server 1 sends out a DHCP Discover packet, it takes longer for DHCP messages from clients to reach the DHCP Server 2 than DHCP Server 1, because DHCP Server 2 is on the other side of a router from the DHCP client. You can also configure a delay on the DHCP relay agent to ensure the local DHCP server has adequate time to respond. Because DHCP clients always accept the lease from the DHCP server that sends the first response, clients normally obtain leases from DHCP Server 1. If DHCP Server 1 goes offline for any reason, clients accept leases from DHCP Server 2.