BGP requires manual configuration of neighbors. Once neighbors are manually configured, it goes through 6 states until it is fully established. Knowing these states would help us determine the stage our connection is currently in. It is also very important in troubleshooting as it helps us understand what went wrong during adjacency.
BGP Neighbor Adjacency States:
1. IDLE – This is normally can be seen if BGP is down / administratively down or just waiting for the next attempt. At this stage, no BGP incoming sessions are permitted.
My BGP is established between Culloden and Stirling sites and Culloden and Fyvie sites. But when I shut down s2/2 link between Culloden and Fyvie, my BGP went to IDLE state.
Culloden(config)#int s2/2 Culloden(config-if)#shut Culloden(config-if)#end