ISC DHCP server config option min-secs
The min-secs statement
Seconds should be the minimum number of seconds since a client began trying to acquire a new lease before the DHCP server will respond to its request. The number of seconds is based on what the client reports, and the maximum value that the client can report is 255 sec- onds. Generally, setting this to one will result in the DHCP server not responding to the client's first request, but always responding to its second request.
This can be used to set up a secondary DHCP server which never offers an address to a client until the primary server has been given a chance to do so. If the primary server is down, the client will bind to the secondary server, but otherwise clients should always bind to the primary. Note that this does not, by itself, permit a primary server and a secondary server to share a pool of dynamically-allocat- able addresses.
Simple idea and easy to implement using Kea pkt4::getSecs.