Dell PowerEdge FN I/O Module Configuration Guide 9.10(0.0)

In order to make decisions in its operations with other BGP peers, a BGP process uses a simple finite state machine that consists of six states: Idle, Connect, Active, OpenSent, OpenConfirm, and Established. For each peer-to-peer session, a BGP implementation tracks which of these six states the session is in. The BGP protocol defines the messages that each peer should exchange in order to change the session from one state to another.

State	Description
Idle	BGP initializes all resources, refuses all inbound BGP connection attempts, and initiates a TCP connection to the peer.
Connect	In this state the router waits for the TCP connection to complete, transitioning to the OpenSent state if successful. If that transition is not successful, BGP resets the ConnectRetry timer and transitions to the Active state when the timer expires.
Active	The router resets the ConnectRetry timer to zero and returns to the Connect state.
OpenSent	After successful OpenSent transition, the router sends an Open message and waits for one in return.
OpenConfirm	After the Open message parameters are agreed between peers, the neighbor relation is established and is in the OpenConfirm state. This is when the router receives and checks for agreement on the parameters of open messages to establish a session.
Keepalive and Established	Keepalive messages are exchanged next, and after successful receipt, the router is placed in the Established state. Keepalive messages continue to be sent at regular periods (established by the Keepalive timer) to verify connections.

After the connection is established, the router can now send/receive Keepalive, Update, and Notification messages to/from its peer.

Peer Groups

Peer Ggroups are neighbors grouped according to common routing policies. They enable easier system configuration and management by allowing groups of routers to share and inherit policies.

Peer groups also aid in convergence speed. When a BGP process needs to send the same information to a large number of peers, the BGP process needs to set up a long output queue to get that information to all the proper peers. If the peers are members of a peer group however, the information can be sent to one place and then passed onto the peers within the group.