GAB waits until the specified number of nodes becomes available to automatically create the port "a" membership. Port "a" indicates GAB membership for an SF Oracle RAC cluster node. Every GAB reconfiguration, such as a node joining or leaving increments or decrements this seed membership in every cluster member node.
Port a gen 7e6e7e01 membership 01
In this case, 7e6e7e01 indicates the "membership generation number" and 01 corresponds to the cluster's "node map". All nodes present in the node map reflects the same membership ID as seen by the following command:
Port a gen 7e6e7e04 membership ;1
When the last and the final system is brought back up from a complete cluster cold shutdown state, the cluster will seed automatically and form port "a" membership on all systems. Systems can then be brought down and restarted in any combination so long as at least one node remains active at any given time.
The fact that all nodes share the same membership ID and node map certifies that all nodes in the node map participates in the same port "a" membership. This consistency check is used to detect "split-brain" and "pre-existing split-brain" scenarios.
Split-brain occurs when a running cluster is segregated into two or more partitions that have no knowledge of the other partitions. The pre-existing network partition is detected when the "cold" nodes (not previously participating in cluster) start and are allowed to form a membership that might not include all nodes (multiple sub-clusters), thus resulting in a potential split-brain.