The cluster status in the Dashboard shows secondary server(s) is(are) down, but actually it (they) is (are) up and running.

The network interfaces of the primary server are on the same network. Both MUST be configured on different networks (Administration -> Network Settings -> Interfaces).

Example:
eth0 -> 192.168.1.1 (255.255.255.0) -> Web Access
eth1 -> 192.168.2.1 (255.255.255.0) -> Cluster Access

For optimal cluster performance and network bandwidth utilization, always configure one of the NICs (for each member of the cluster) on dedicated network. That way, the cluster traffic gets split from the email traffic that goes over SMTP which eventually offloads the eth0 interfaces.

NOTE:   Cluster traffic is quite intensive when the servers are configured in local cluster vs. global cluster. This is due to the amount of information that gets exchanged to keep the nodes synchronized.