Openwrt port mirroring

OpenWrt device with port mirroring enabled to act as if it is between the opnsense device and the ISP device

Internet
   |
[ ISP device ]
   |  (WAN link)
   | eth0
   |
+------------------------+
|   OpenWrt switch/hw    |
|  (port-mirroring on)   |
|                        |
|  Port A  <-----> Port B|
|  (to ISP)    (to OPNsense WAN)
|      \                 |
|       \                |
|        \               |
|      [mirror]          |
|         |              |
|     Port M (monitor) <-+  <-- capture machine (tcpdump/wireshark)
+------------------------+
ISP modem  --- port 1 (switch) ---+
                                   \
                                    \       (traffic flows between port1<->port2)
                                     +--- port 2 (switch) --- OPNsense WAN
                                     |
                                     +--- port 8 (switch, monitor) --- Capture PC (eth0)
                                     (port 1 or port 2 mirrored => port 8)