ISA 2004 Satellite Connection Problem Pls Help

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Pls help me, I have one ISA2004 installed in windows 2003 with 2 NIC cards
one for external and one for internal LAN, oUr internet is thru Satellite
connection, My one NIC external card was directly connected to the satellite
device that acts as router, my problem is that my ISA2004 external card can't
get ip address from our ISP, and i already configured my external card to be
a dhcp client and still can not
get Ip address, but before that when only windows 2003 was installed on
machine it's getting ip from ISP, it's only after i installed the ISA2004
that problem exist, It's seems that ISA2004 is blocking or not allowing
incoming dhcp packet from ISP going to my external LAN, Now how can i allow
my ISA2004 to get ip
address from our ISP, or what was really the cause of not getting ip
address, Pls kindly help me about this problem, Thank you very much in
advanced..
.