Server 2008 R2 provides an easy way to split DHCP scopes per best practices. Best practices normally dictate allocating 80 percent of the address scope to the primary DHCP server and 20 percent to a second DHCP server which, which would be configured with a delay in responding to address lease requests. With this configuration, clients will usually get an address from the primary DHCP server.
To use the split scope, first create the DHCP scope as normal and then select Split-Scope from the Advanced menu of the scope.
This will launch the Split-Scope. Click Next to go to the main screen, which allows you to select the second DHCP server that will take over a portion of the scope.
Configure which IP addresses to allocate to the second DHCP server, either by adjusting the percentages or by directly setting the IP addresses. Click Next once set.
The next screen allows you to configure the delay, in milliseconds, for the DHCP servers. This is where you would set a delay on the additional DHCP server that has been added so the primary is used for requests, if it’s available.
After clicking Next, a summary will be displayed and the scope split. You’ll notice that exclusions are added on each DHCP server for the addresses that are active on the other DHCP server.