Setup a UDP heartbeat on an interface. UDP heartbeat is used to synchronize
two HA units. Do not run both UDP and multicast heartbeat on the same interface.
You should only use one of them. Only use a secure interface for synchronization.
Multicast is probably the most used way of synchronization rather than using
UDP. You are getting better control with Multicast. Note, do not forget to
setup a rule for udp synchronization in your incoming and outgoing rulebase.