@JKnott said in What does WAN monitoring do?:
If the WAN fails, what mechanism is there, in pfSense, to force change?
It will change the default gateway (default route) and hence anything using that. If you have set the default gateway to a failover group it will use the gateways from within that. If it's set to automatic is just uses the next gateway that is UP which can be an issue is that is, say, a VPN.
If you're policy routing traffic you can set a load-balance or failover gateway group and pf forces traffic via that as states are opened. If a gateway is marked down it is removed from the group.