[dantro]

Sounds like you have an anti-spoofing issue.
Create a new network object with your .101 network. Create a group and put this network into it. Also place the network object with your .100 network into this group. Go to your firewall object and edit its properties. In 'Topology', edit the .100 interface > topology and use your new group to specify the IP-adresses behind this internal interface.