DHCP multicast/unicast

[Emsi at gmx.de]

Hi,

I'm running a large switched network, where we separate each port via a 
3com feature called port isolation, which puts every single port in a 
"vlan" for security purposes. Unfortunately I have seen with a sniffer on 
a users end-port, that you can see DHCPOFFERs which belong to other users on other ports, cause isc-dhcpd-V3.0.4 is sending the offers as link layer 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? Does 
the parameter "shared-network" influences this behaviour? How can I force 
ISC DHPC to answer with unicast (link layer)?

thanks!
Oliver