problem with isa server and dhcp spoofed ip lease

I recently installed internet security and acceleration server on my Win2k
server machine.
I was using the standard Routing and remote access in NAT mode.
I use a modem that spoofd my ip to the ethernet card so that the modem
apears to be transparent.

this option worked fine under de routing and r.....
I got the ip spoofed lease for 10 minutes,when te lease was in use for
over 5minutes it was refresht and another 10minutes was added to that
lease.this went on a long as the modem and server stayed online.

Now when i installed ISA:
I got the lease for again 10minutes but after 5minutes,when i normally
receive a refresh ,it stayed.Until the 10minute lease was over resulting
in the loosing of the ip lease -> NO INTERNET CONNECTION.

MY Q IS NOW , am i blocking the traffic for a refresh of dhcp spoofed
lease,which protocol and port should I use.
WHAT AM I DOING WRONG????

PLEASE HELP!!

sorry for my language ,i am dutch!

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