When installing Oracle 12c RAC, the Grid Installer fails with:

Specified network interface doesn't maintain connectivity across cluster nodes.

 
This error is caused when the Linux firewall is still operational.

Log in as root, shutdown and disable the firewall on all nodes of the cluster:

[root@ora12a ~]# service iptables stop

iptables: Flushing firewall rules:                         [  OK  ]
iptables: Setting chains to policy ACCEPT: filter          [  OK  ]
iptables: Unloading modules:                               [  OK  ]

[root@ora12a ~]# chkconfig iptables off

 
Now retry the Grid installer step and it should succeed.