As dhcp requests are broadcast from the client to any server which is listening, if there isn't a dhcp server on the local subnet, the request will fail. The way this is overcome in the Cisco World is to direct broadcast requests for dhcp to a specified ipaddress, effectively converting a broadcast to a unicast. The router then sends the reply containing the allocated ipaddress back to the client.

An IOS example for BCC is:-

!
interface Vlan30
 description *** site name ***
 ip vrf forwarding network1
 ip address 172.16.nnn.nnn 255.255.255.nnn
 ip helper-address 172.16.123.123
!