Having the following configuration:
interface X
ip helper-address 1.1.1.101
ip helper-address 1.1.1.102
This will configure DHCP relay in the router. It receives a broadcast DHCP request and converts it into a unicast message directed exclusivly to both DHCP servers.
The router "sees" the DHCPDISCOVER packet and forwards it to bothaddresses simultaneously. Then, both DHCP servers will make an offer (DHCPOFFER), if they are up and received the request.
The client will receive each offer at one time, one first then the other. if it finds the offer agreeable, itwill send another broadcast, a DHCPREQUEST, specifically requestingthose particular IP parameters. Why does the client broadcast therequest instead of unicasting it to the server? A broadcast is usedbecause the first message, the DHCPDISCOVER, may have reached more thanone DHCP server. If more than one server makes an offer, thebroadcasted DHCPREQUEST allows the other servers to know which offerwas accepted. The offer accepted is usually the first offer received.
Sometimes, in other to make one server responde faster then the other, you can configure the delay of response in the DHCP server.Also, for sincronization between DCHP server, either they support this feature or you will have to have your subnet divided betweenthe 2 servers without overlapping.
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