How to configure a BIPAC-7402G when conencting to firewall?

[timcunnell]

Hey guys - I hope someone may be able to help with this one.
I am managing a few sites where we are using BIPAC-7402G routers. I want to now connect these routers to dedicated firewalls, and just use the Billion devices for the ADSL conenction, passing everything else to the firewall. The problem is that when I set up the devices like this, I am getting all kinds of strange conenctivity problems. For example, some users (though not all) lose Exchange conenctivity, I can't access firewalls through internet browser even though they respond to a ping, RDP either doesn't work or is very slow.
I have the Billion routers configured with a fixed IP address on the WAN side for ADSL, then LAN is set to 172.16.0.1. I then set the WAN side of the firewalls to 172.16.0.2 and internal LAN is 192.168.x.x. I have turned the firewall setting off on the Billion routers, and also used Virtual Server to forward ports 500 and 4500 for IKE to 172.16.0.2 in order to enable IPSEC VPN, which is managed by the firewalls. With all this in place, everything pings great accross different sites. But end users are having tons of problems.
I suspect this is maybe a NATing issue? But I don't really know where to start looking. I guess I need the router to just forward all traffic to the WAN side of the firewall, but presumably this isn't happening.
Any advice or suggestions would really be appreciated. Thanks in advance for any help!
Cheers
Tim

[admin]

Maybe you can just enable dmz on your 7402G pointing it to your firewall so everything should go straight to it.