OPNsense as secondary router and common LAN: how to configure LAN interface?

[cmonty14]

Hello,
I have a network setup with 2 routers.
OPNsense is the secondary router.
It's WAN interface is connected to primary router's downstream interface.

Now I want to use a single LAN interface for administration, means primary router's LAN and secondary router's LAN are the same subnet.

Here's an overview of the IP adresses:
Primary router:
  WAN: <public IPv4>
  LAN: 172.16.1.1/24
  NET: 172.16.0.1/29
Secondary router:
  WAN: 172.16.0.2/29
  LAN: 172.16.1.2/24

In my understanding any traffic on subnet 172.16.1.0/24 (LAN) must be routed to primary router only.

How can I ensure that OPNsense is not routing LAN traffic over it's WAN interface?

THX