Cluster Nodes Can Connect to the Database, but Fail to Make a Cluster
This article applies to Confluence clustered 5.4 or earlier.
The typical cluster related fatal error occurs:
Fatal error in Confluence cluster: Database is being updated by an instance which is not part of the current cluster. You should check network connections between cluster nodes, especially multicast traffic.
In this case, even if the hosts are connected by crossover ethernet cable (so routing, router or switch issues are not possible) and the hosts can ping each other across the cable, the nodes are not able to make a cluster. Increase logging to DEBUG level - Confluence performs some UDP multicast testing. You will need to test that UDP traffic is allowed in the network.
UDP multicast is not being permitted across the network interface.
Reconfigure the interface to permit UDP multicast. Alternatively, consider configuring a cluster for unicast traffic.