Before any routing information is transmitted, BGP establishes a neighbour adjacency with other routers, similar to OSPF (Open Shortest Path First) or EIGRP (Enhanced Interior Gateway routing Protocol). BGP, on the other hand, does not use broadcast or multicast for BGP neighbour discovery; instead, neighbours are manually configured and communicate over TCP/179.

Two BGP systems will go through a succession of BGP Neighbor Adjacency States before becoming neighbours. They are as follows:

• Idle - The router initialises BGP resources. Attempts to establish a BGP incoming connection are rejected. The BGP protocol establishes a TCP connection with the peer.
• Connect - BGP waits for the THREE way handshake to finish. The OPEN message is transmitted to the peer if it is successful, and BGP shifts to the OpenSent state. If we are unsuccessful, we will return to the Active state. If the ConnectRetry timeout expires, however, BGP will continue in this condition, with the timer reset and a new three way handshake launched.
• Active - BGP RETURNS to the Connect state after the ConnectRetry timeout is reset.
• OpenSent - BGP waits for an OPEN message from its peer before sending it. BGP enters the OpenConfirm state after receiving a message.
• OpenConfirm - BGP waits for a peer to send a keepalive message. BGP goes to the Established state if a response is received before the timeout ends. Otherwise, BGP SWITCHES to Idle mode.
• Established - Both peers exchange UPDATE messages once the connection is established. If any of the UPDATE messages include an error, the BGP peer will send a NOTIFICATION message and enter the Idle state.