If some of the nodes are not up and running in a cluster, then GAB port does not come up to avoid any risks of preexisting split-brain. In such cases, you can manually seed GAB using the command gabconfig - x to bring the GAB port up.
However, if you have enabled I/O fencing in the cluster, then I/O fencing can handle any preexisting split-brain in the cluster. You can configure I/O fencing in such a way for GAB to automatically seed the cluster. The behavior is as follows:
If a number of nodes in a cluster are not up, GAB port (port a) still comes up in all the member-nodes in the cluster.
If the coordination points do not have keys from any non-member nodes, I/O fencing (GAB port b) also comes up.
This feature is disabled by default. You must configure the autoseed_gab_timeout parameter in the
/etc/vxfenmode file to enable the automatic seeding feature of GAB.
To enable GAB auto-seeding parameter of I/O fencing
/etc/vxfenmodefile to 0 to turn on the feature.
To delay the GAB auto-seed feature, you can also set a value greater than zero. GAB uses this value to delay auto-seed of the cluster for the given number of seconds.