Assigned Interfaces
While not strictly necessary, it is possible to assign individual interfaces for OpenVPN servers and clients alike. However doing so may yield unexpected behaviour of firewall rules. Most notably, rules created on an assigned interface of an OpenVPN Roadwarrior server are created with the reply-to directive by default, which breaks client connectivity.
There are multiple ways to fix this problem. For most setups, it will be sufficient to disable the automatically created IPv4 and IPv6 Gateways under System › Gateways › Configuration. Doing so will also disable the automatic addition of the reply-to directive to rules created on the interface, and client connectivity will be restored.
Another option is to manually select the option “Disable Reply-To” on each firewall rule you generate on the assigned interface. See Firewall for further details.
The third option is to globally disable the generation of reply-to completely as described in Firewall settings. However this method can break Multi-WAN setups.