I run a transparent OPNsense bridge between a UniFi Dream Machine Pro and the rest of my LAN. It is deliberately boring at Layer 3: the UDM keeps routing, DHCP, DNS, firewall policy, WAN handling, and VLAN definitions. OPNsense sits inline as a Layer 2 bump in the wire.
The interesting part is that both sides of that bump use LACP.
I already wrote the build/configuration guide for this setup here: Building a Transparent LAGG (LACP) Bridge with OPNsense, UDM, and UniFi - A Practical Guide. That article explains how the bridge was built, how the LAGG devices were configured, and why I wanted the firewall to remain transparent.
This article is the other half of the story: what happens when that kind of setup fails in a non-obvious way.
Not a clean outage. Not a single "the network is down" moment. Just enough instability to make everything feel wrong.
