6 BGP States

The Border Gateway Protocol (BGP) is a crucial component of the internet's infrastructure, enabling the exchange of routing information between autonomous systems (ASes). One of the key aspects of BGP is its finite state machine, which defines the various states a BGP session can be in. Understanding these states is essential for designing, implementing, and troubleshooting BGP networks. In this article, we will delve into the six BGP states, exploring their characteristics, transitions, and implications for network operation.

Introduction to BGP States

BGP states are the conditions or statuses that a BGP session can be in at any given time. These states reflect the progress and health of the BGP session establishment and operation. The six BGP states are: Idle, Connect, Active, OpenSent, OpenConfirm, and Established. Each state represents a specific phase in the BGP session lifecycle, from initial connection attempts to the full exchange of routing information.

BGP State 1: Idle

The Idle state is the initial state of a BGP session. In this state, the BGP speaker is not actively attempting to establish a connection with its peer. The speaker may be waiting for a start event, such as the establishment of a TCP connection, or it may be recovering from a previous connection failure. The Idle state is a passive state, where the BGP speaker is not sending any messages to its peer.

BGP State 2: Connect

The Connect state is entered when the BGP speaker initiates a TCP connection to its peer. This state involves the establishment of a transport connection between the two BGP speakers. If the TCP connection is successful, the BGP speaker transitions to the OpenSent state. However, if the connection attempt fails, the speaker returns to the Idle state.

BGP State 3: Active

The Active state is similar to the Connect state, with the BGP speaker attempting to establish a connection with its peer. However, in the Active state, the speaker has previously attempted to connect and has returned to this state after a failure. The Active state is essentially a retry mechanism, allowing the BGP speaker to periodically attempt to reconnect with its peer.

BGP State 4: OpenSent

In the OpenSent state, the BGP speaker has successfully established a TCP connection with its peer and has sent an OPEN message. The OPEN message contains information about the BGP speaker, such as its AS number, hold time, and supported optional parameters. If the peer accepts the OPEN message, it responds with its own OPEN message, and the BGP speaker transitions to the OpenConfirm state.

BGP State 5: OpenConfirm

The OpenConfirm state is entered when the BGP speaker receives a valid OPEN message from its peer. In this state, the speaker waits for a KEEPALIVE message from its peer, which confirms the establishment of the BGP session. If the KEEPALIVE message is received, the BGP speaker transitions to the Established state. However, if the KEEPALIVE message is not received within the specified hold time, the speaker returns to the Idle state.

BGP State 6: Established

The Established state is the final state of a BGP session, where the two BGP speakers have successfully exchanged OPEN messages and have established a stable connection. In this state, the speakers can exchange UPDATE messages, which contain routing information. The Established state represents a fully operational BGP session, where the speakers can advertise and learn routes from each other.

In conclusion, the six BGP states provide a comprehensive framework for understanding the lifecycle of a BGP session. By recognizing the characteristics and transitions of each state, network administrators can design, implement, and troubleshoot BGP networks more effectively. The BGP states serve as a foundation for establishing and maintaining stable BGP sessions, which are critical for the exchange of routing information between autonomous systems.