DHCP Relay through another firewall

Hello,

I have set up two ISA Servers in my lab (learning environment).

One faces the internet, the other one is in between the lan and the perimetral network.

Sort of:

LAN  <--- ISA2--->  DMZ  <---ISA1---> INTERNET.

The thing is that the other office connects to ISA1 through a VPN-Site-to-Site, and it goes all fine except that ISA1 cannot take any ip from the dhcp server standing on the lan.

I have read this: http://technet.microsoft.com/en-us/library/cc302680.aspx

But in my lab, there are two isa servers , so I don't know how to send dhcp requests from isa1 to isa2 excepto for broadcast 255.255.255.255 , but the isa1 does not know how to forward that to the dhcp server in the lan.

I am mixed up because I am not an expert and am learning now about this dhcp relay thing now.

Thanks in advance!!

Luis Olías Técnico/Admon Sistemas . Sevilla (España - Spain)