When does BGP enter the Idle state?

BGP enters the Idle state when the peer sends a NOTIFICATION message. This state indicates that BGP is unable to establish or maintain a connection with its peer due to an error condition detected by the peer that triggers the NOTIFICATION message. The NOTIFICATION message conveys specific issues, such as problems with capability negotiation or session rejection, prompting BGP to transition to the Idle state to halt further attempts for the time being.

Understanding the Idle state is crucial for troubleshooting BGP sessions. In contrast, BGP operates in other states—such as Connect, Active, OpenSent, or Established—under different conditions. For instance, BGP moves to the Established state after successfully exchanging OPEN messages, which establishes a connection but does not relate to entering the Idle state. Similarly, resources being initialized or receiving keepalive messages would not trigger the transition to Idle, as those pertain to establishing or maintaining an active session instead.