Just to give a little of an explanation with that, DHCP discovery messages are initially sent via broadcasts out by the client looking for a DHCP server. As we know, routers stop broadcasts in there tracks. In order to fix this issue, you configure the interface where the DHCP discovery broadcasts would be coming in with the ip helper-address to "convert" the broadcasts into a more accurately directed unicast towards the DHCP server. After discovery, the offer, request and acknowledgements messages of DHCP are all sent as unicast messages.
Sent from Cisco Technical Support iPhone App
