Border Gateway Protocol – Finite State Machine (BGP-FSM)

Introduction

The BGP-FSM model presents different states during BGP neighbor negotiation: Idle, Connect, Active, OpenSent, OpenConfirm and Established. It also describes the events that launch changes from one state to another. This article is based on Based on RFC 4271 – A Border Gateway Protocol 4 (BGP-4).

There are 28 events, which are grouped into four categories.

• Administrative Events: Events that either start or stop the process.
• Timer Events: Expiration of HoldTimer, KeepaliveTimer or ConnectRetryTimer trigger this event.
• TCP connection based events: These events describe the different state of the TCP three-way handshake.
• BGP message based events: Receiving or sending BGP messages: OPEN, KEEPALIVE, UPDATE and NOTIFICATION launch this event.

A Complete list can be found in Appendix A.

Figure 1-1 shows the map of different states and related event numbers. Timers and counters are excluded from both the text part and picture to keep things as clear as possible.

Figure 1-1: Overview of BGP-FSM.