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All processes in SF Oracle RAC use GAB for communication. Each process wanting to communicate with a peer process on other nodes registers with GAB on a specific port. This registration enables communication and notification of membership changes. For example, the VCS engine (had) registers on port h. had receives messages from peer had processes on port h. had also receives notification when a node fails or when a peer process on port h becomes unregistered.
Some processes use multiple ports for specific communications requirements. For example, CVM uses multiple ports to allow communications by kernel and user-level functions in CVM independently.