Proper seeding of the cluster and the use of low priority heartbeat cluster interconnect links are best practices with or without the use of I/O fencing. Best practice also recommends multiple cluster interconnect links between systems in the cluster. This allows GAB to differentiate between:
A loss of all heartbeat links simultaneously, which is interpreted as a system failure. In this case, depending on failover configuration, HAD may attempt to restart the services that were running on that system on another system.
A loss of all heartbeat links over time, which is interpreted as an interconnect failure. In this case, the assumption is made that there is a high probability that the system is not down, and HAD does not attempt to restart the services on another system.
In order for this differentiation to have meaning, it is important to ensure the cluster interconnect links do not have a single point of failure, such as a network switch or Ethernet card.