It seems your question is difficult to answer without first asking another question. But let us try this:
It seems you wanted to trap all packets and make IP 172.X.X.X your gateway, so try it this way:
Code:
~$ /sbin/route add -net 0.0.0.0 netmask 0.0.0.0 gw 172.X.X.1 p3p1
Then check if you have made it a gateway:
If it doesn't work come back and give us more about what you have and what you wanted to happen.
Hope that helps.
malek mustaqiim