DHCP and Multicast/Unicast

Hi,

I'm running a large switched network, where we separate each port in an
individual VLAN for security purposes. Unfortunately I have a seen with
a sniffer on a users port, that he can see DHCPOFFERs belong to other
users on other ports, cause isc-dhcpd-V3.0.4 is sending the Offer as
broadcast to ff:ff:ff:ff:ff:ff. I read following in a book about that
topic:

Note: The Internet Software Consortium (ISC) DHCP server cheats when it
can by setting the IP destination address to 255.255.255.255 and the
link layer destination address to the client's link-layer address. This
is technically incorrect, but it works well and reduces the amount of
broadcast traffic generated by the DHCP server."

Under which conditions does ISC-DHCPD answer with unicast offers? Doese
the paramater shared-network influences this behaviour? How can I force
ISC DHPC always to answer unicast (link layer) on DHCPDISCOVERS?

Thanks for your help!

regards
Oliver