The client send a broadcast for bootp and the broadcast will only reach till the layer 3 interface where multiple helper address is configured bur after that it will be a unicast to multiple DHCP addresses whatever is configured in ip helper command and and then the DHCP request will be sent to each and every DHCP server what is configured in helper address command and all the servers will reply and it depends on client which one to take.
What exactly happens is mostly DHCP server are centrally located in one vlan and clients may exist in multiple vlan. The broadcast for DHCP will not cross the layer 3 boundary so "ip helper-address " is configured in respective vlan layer 3 interfaces. Once the broadcast for bootp hits the layer 3 interface is converted as a unicast request and sent to the DHCP server configured in ip helper command and then the DHCP server send a DHCP offer packet which is then again forwarded back to the client.
Then client again sends the DHCP request and gets an acknoledgement.
I just noticed that Ankur has responded to your query before I did. I would just point out one thing in his posting, there's a perception that the ip helper address would always use a unicast DHCP server address but it doesn't have to be that way. The helper address can be configured to point to a whole subnet, a directed broadcast rather than a unicast, and this can be useful when the unicast IP of the server is not known or when multiple servers exist in a remote subnet.
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