Stateful Switchover (SSO) provides protection for network edge devices with dual Route Processors (RPs) that represent a single point of failure in the network design, and where an outage might result in loss of service for customers.
Reference: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/12-
2SY/configuration/guide/sy_swcg/stateful_switchover.html

The BGP session may report in the following states
1 - Idle: the initial state of a BGP connection. In this state, the BGP speaker is waiting for a BGP start event, generally either the establishment of a TCP connection or the re-establishment of a previous connection. Once the connection is established, BGP moves to the next state.
2 - Connect: In this state, BGP is waiting for the TCP connection to be formed. If the TCP connection completes, BGP will move to the Open Sent stage; if the connection cannot complete, BGP goes to Active
3 - Active: In the Active state, the BGP speaker is attempting to initiate a TCP session with the BGP speaker it wants to peer with. If this can be done, the BGP state goes to Open Sent state.
4 - Open Sent: the BGP speaker is waiting to receive an OPEN message from the remote BGP speaker
5 - Open Confirm: Once the BGP speaker receives the OPEN message and no error is detected, the BGP speaker sends a KEEPALIVE message to the remote BGP speaker
6 - Established: All of the neighbor negotiations are complete. You will see a number, which tells us the number of prefixes the router has received from a neighbor or peer group.