In its simplest form, this is the graceful shutdown procedure. Details and exceptions to this procedure when there are active calls or registrations are discussed in later paragraphs. The first six actions are performed whether or not the SBC is part of an Oracle Communications Subscriber-Aware Load Balancer (Subscriber-Aware Load Balancer) Cluster

• The SBC receives the set-system-state offline command.
• The SBC transitions to becoming offline mode.
• The SBC accepts calls and subscribes from registered endpoints.
• The SBC rejects calls from non-registered endpoints.
• The SBC rejects new registrations with a 503 Service Unavailable error message.
• The SBC checks the number SIP INVITE based sessions and number of SIP registrations. When both counts are 0, the SBC transitions to the offline state.

If the SBC is part of an Subscriber-Aware Load Balancer Cluster:

• The Subscriber-Aware Load Balancer client on the SBC changes its cluster status to shutdown state.
• The SBC informs the Subscriber-Aware Load Balancer that it is offline.
• The Subscriber-Aware Load Balancer ceases to forward new end-points to the SBC and puts the SBC in a shutdown state.
• Subscriber-Aware Load Balancer continues to forward all messages for existing registered endpoints to the offline SBC.
• The SBC continues to send heartbeat updates the Subscriber-Aware Load Balancer as before.