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All ip helper-address lines configured in your VLAN take the DHCP broadcast from the client, add the router's (gateway) address into the UDP packet, then unicasts to the DHCP servers. I'm sure the packet rewrite is only done once, then a copy sent to each DHCP server. DHCP IP Helper addresses are IP addresses configured on a routed interface such as a VLAN Interface or a routers Ethernet interface that allows that specific device to act as a “middle man” which forwards BOOTP (Broadcast) DHCP request it receives on an interface to the DHCP server specified by the IP Helper address via unicast.