We are going to make several assumptions in this document. If you don’t have these things done already you will need to go get them done before L2TP will work correctly.
Your firewall is already setup to do basic NAT and you have tested this, or at least it is doing what ever kind of routing you wanted it to do.
You have configured at least one interface on the firewall so it is working and:
The Client Machine(s) can route to (access) one of the interfaces of your firewall. Make sure of this. If it is an interface that you allow ICMP to access I suggest pinging it.
You have a client machine running some form of VPN client that supports L2TP.
Now that we have the basics let’s get started on the firewall settings.