BGP finite state machine

States

Each configured peer has an independent FSM instance. The six states:

State	Description
Idle	No resources allocated. Refuses all incoming connections. Waits for ManualStart or AutomaticStart.
Connect	TCP connection attempt in progress. ConnectRetryTimer running.
Active	Listening for incoming TCP connection (passive mode). ConnectRetryTimer running.
OpenSent	TCP connected. OPEN message sent. Waiting for peer’s OPEN.
OpenConfirm	OPEN received and validated. KEEPALIVE sent. Waiting for peer’s KEEPALIVE.
Established	Session fully operational. UPDATE and KEEPALIVE messages exchanged.

Session attributes

Mandatory

Every FSM instance maintains:

Attribute	Purpose
State	Current FSM state
ConnectRetryCounter	Number of connection attempts
ConnectRetryTimer	Time before retrying TCP connection
HoldTimer	Deadman timer — peer must send KEEPALIVE/UPDATE before expiry
HoldTime	Negotiated hold interval (initial value from configuration)
KeepaliveTimer	Interval for sending KEEPALIVE messages
KeepaliveTime	Initial value (typically HoldTime/3)

Optional

Attribute	Purpose
DampPeerOscillations	Back off when peer flaps repeatedly
IdleHoldTime / IdleHoldTimer	Hold in Idle state after flap (exponential backoff)
DelayOpen / DelayOpenTimer	Delay sending OPEN to allow peer to go first
PassiveTcpEstablishment	Listen only, do not initiate TCP
SendNOTIFICATIONwithoutOPEN	Allow NOTIFICATION before OPEN exchange
CollisionDetectEstablishedState	Process collisions in Established state

Events

Events drive state transitions. They fall into five categories.

Administrative events (1-8)

Event	Name	Status
1	ManualStart	Mandatory
2	ManualStop	Mandatory
3	AutomaticStart	Optional
4	ManualStart with PassiveTcpEstablishment	Optional
5	AutomaticStart with PassiveTcpEstablishment	Optional
6	AutomaticStart with DampPeerOscillations	Optional
7	AutomaticStart with DampPeerOscillations and PassiveTcp	Optional
8	AutomaticStop	Optional

Timer events (9-13)

Event	Name	Status
9	ConnectRetryTimer_Expires	Mandatory
10	HoldTimer_Expires	Mandatory
11	KeepaliveTimer_Expires	Mandatory
12	DelayOpenTimer_Expires	Optional
13	IdleHoldTimer_Expires	Optional

TCP events (14-18)

Event	Name	Status
14	TcpConnection_Valid	Optional
15	Tcp_CR_Invalid	Optional
16	Tcp_CR_Acked	Mandatory
17	TcpConnectionConfirmed	Mandatory
18	TcpConnectionFails	Mandatory

BGP message events (19-28)

Event	Name	Status
19	BGPOpen	Mandatory
20	BGPOpen with DelayOpenTimer running	Optional
21	BGPHeaderErr	Mandatory
22	BGPOpenMsgErr	Mandatory
23	OpenCollisionDump	Optional
24	NotifMsgVerErr	Mandatory
25	NotifMsg	Mandatory
26	KeepAliveMsg	Mandatory
27	UpdateMsg	Mandatory
28	UpdateMsgErr	Mandatory

State transitions in detail

Idle state

Connect state

TCP initiation is in progress. Three outcomes:

1. TCP succeeds (Event 16/17) — clear ConnectRetryTimer, send OPEN, start HoldTimer → OpenSent
2. TCP fails (Event 18) — restart ConnectRetryTimer, continue listening → Active
3. ConnectRetryTimer expires (Event 9) — drop TCP, restart timer, initiate new TCP → Connect

Any error or ManualStop → close connection, release resources → Idle

Active state

Passively waiting for inbound TCP. Key transitions:

1. TCP succeeds (Event 16/17) — clear ConnectRetryTimer, send OPEN → OpenSent
2. ConnectRetryTimer expires (Event 9) — attempt active TCP connection → Connect
3. TCP fails (Event 18) — restart timer, increment counter → Idle

OpenSent state

OPEN sent, waiting for peer’s OPEN. The large initial HoldTimer (4 minutes) runs.

1. Valid OPEN received (Event 19) — negotiate Hold Time, set HoldTimer, send KEEPALIVE → OpenConfirm
2. Error in OPEN (Event 22) — send NOTIFICATION → Idle
3. TCP fails (Event 18) — close, restart ConnectRetryTimer → Active
4. Connection collision (Event 23) — resolve per BGP Identifier comparison

OpenConfirm state

Both OPENs exchanged. Waiting for the confirming KEEPALIVE.

1. KEEPALIVE received (Event 26) — start negotiated HoldTimer → Established
2. NOTIFICATION received (Event 24/25) → Idle
3. HoldTimer expires (Event 10) — send NOTIFICATION (Hold Timer Expired) → Idle

Established state

Session operational. Normal processing:

• UpdateMsg (Event 27) — process UPDATE, restart HoldTimer
• KeepaliveTimer expires (Event 11) — send KEEPALIVE
• KeepAliveMsg (Event 26) — restart HoldTimer
• HoldTimer expires (Event 10) — send NOTIFICATION → Idle
• UpdateMsgErr (Event 28) — send NOTIFICATION → Idle
• ManualStop (Event 2) — send Cease NOTIFICATION → Idle

Connection collision detection

When both peers initiate TCP simultaneously, two connections form. Resolution:

Collision detection happens upon receiving an OPEN when another connection to the same peer is in OpenConfirm state. The loser gets a Cease NOTIFICATION.

Peer oscillation damping

Implementations track the ConnectRetryCounter. After repeated flaps, the IdleHoldTime increases (commonly doubling up to a cap like 120 seconds). This prevents tight reconnection loops from consuming resources across the network.