The HA (high availability) cluster solution consists of having two computers with replicated configurations playing different roles; active and standby.
With this configuration, neither of the two SBCs assumes the role of the main team and the backup team, but rather the active / standby role, sharing the same virtual IPs.
The SBCs maintain communication about the health level of each one, which allows that, when problems such as an interface drop, power problems, fan problems, etc. are detected in the active equipment, the service would automatically switch to the other. SBC avoiding loss of service.