veroxii , 9 months ago

You can still do this but as others have said you need to have 2 separate lans. Your old Lan can go to PC 1 from old router. Then opnsense wan goes to your existing Lan and importantly you are now creating a new Lan on the lan side of opnsense. Here you can connect the PC 2 to test with. Each PC should only be on 1 Lan and each Lan should have a separate subnet.

See this post and the last comment even references a diagram to exactly what you want: forum.opnsense.org/index.php?topic=32774.0

There are all kinds of routing protocols and algorithms at play which don’t like loops and multiple routers competing to control the same subnet.