This document covers configuration of a High Availability cluster in pfSense® Plus software using the following features:
CARP for IP address redundancy
XMLRPC for configuration synchronization
pfsync for state table synchronization
With this configuration, two units act as an “active/passive” cluster with the primary node working as the master unit and the secondary node in a backup role, taking over as needed if the primary node fails.
The documentation for pfSense software contains additional details: