I'm trying to build a network configuration where clients can connect to some servers in a VPN via a Zywall USG 200 router/firewall.
The architecture is composed by a windows server 2013 that acts as VPN gateway and some other servers connected directly to it via IPSEC/l2tp with PSK. Some computers in a LAN should be able to connect to these servers without connect directly to the VPN itself. To my somewhat limited networking knowledge this could be accomplished by connecting the Zywall to the VPN and then setup some routing (and firewall) rules to enable the clients to reach the VPN network.
What I'm failing to accomplish at the moment, It's how to connect the Zywall as client in the VPN. There are plenties of guides on how to setup the Zywall as a IPSEC/l2tp gateway, but I found none that explains my configuration. From the Zywall USG200 manual I found out how to configure an IPSEC VPN, but I don't understand how to configure the L2TP tunnel. The L2TP page on the Zywall configuration seems used to configure a L2TP "server" ("L2TP Network Server"? I'm a bit confused about nomenclature here) and not the connection to an existing L2TP "server".