I think the real question is why doesn't it work. We want to route the 
traffic without NAT since on the destination network they must be able 
to see all our internal IP addresses.

-Michael

> Did you setup nat rules for the 172.X network? If not it will not work. I have this setup running
in several places and it does work, But you have to use the enable advanced outbound nat and built
the nat tables for the 192 and 172 networks.