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BGP Fundamentals

Chapter Description

In This sample chapter from Troubleshooting BGP: A Practical Guide to Understanding and Troubleshooting BGP, the authors cover BGP Messages and Inter-Router Communication, Basic BGP Configuration for IOS, IOS XR, and NX-OS, IBGP Rules, EBGP Rules, and BGP Route Aggregation

BGP Neighbor States

BGP forms a TCP session with neighbor routers called peers. BGP uses the Finite State Machine (FSM) to maintain a table of all BGP peers and their operational status. The BGP session may report in the following states:

  • Idle

  • Connect

  • Active

  • OpenSent

  • OpenConfirm

  • Established

Figure 1-2 displays the BGP FSM and the states in order of establishing a BGP session.


Figure 1-2 BGP Finite State Machine


This is the first stage of the BGP FSM. BGP detects a start event, tries to initiate a TCP connection to the BGP peer, and also listens for a new connect from a peer router.

If an error causes BGP to go back to the Idle state for a second time, the ConnectRetryTimer is set to 60 seconds and must decrement to zero before the connection is initiated again. Further failures to leave the Idle state result in the ConnectRetryTimer doubling in length from the previous time.


In this state, BGP initiates the TCP connection. If the 3-way TCP handshake completes, the established BGP Session BGP process resets the ConnectRetryTimer and sends the Open message to the neighbor, and then changes to the OpenSent State.

If the ConnectRetry timer depletes before this stage is complete, a new TCP connection is attempted, the ConnectRetry timer is reset, and the state is moved to Active. If any other input is received, the state is changed to Idle.

During this stage, the neighbor with the higher IP address manages the connection. The router initiating the request uses a dynamic source port, but the destination port is always 179.

Example 1-1 shows an established BGP session using the command show tcp brief to display the active TCP sessions between routers. Notice that the TCP source port is 179 and the destination port is 59884 on R1, and the ports are opposite on R2.

Example 1-1 Established BGP Session

RP/0/0/CPU0:R1# show tcp brief | exc "LISTEN|CLOSED"
   PCB     VRF-ID     Recv-Q Send-Q Local Address    Foreign Address      State
0x088bcbb8 0x60000000      0      0     ESTAB
R2# show tcp brief
TCB       Local Address               Foreign Address             (state)
EF153B88 59884                 ESTAB


In this state, BGP starts a new 3-way TCP handshake. If a connection is established, an Open message is sent, the Hold Timer is set to 4 minutes, and the state moves to OpenSent. If this attempt for TCP connection fails, the state moves back to the Connect state and resets the ConnectRetryTimer.


In this state, an Open message has been sent from the originating router and is awaiting an Open message from the other router. After the originating router receives the OPEN message from the other router, both OPEN messages are checked for errors. The following items are being compared:

  • BGP Versions must match.

  • The source IP address of the OPEN message must match the IP address that is configured for the neighbor.

  • The AS number in the OPEN message must match what is configured for the neighbor.

  • BGP Identifiers (RID) must be unique. If a RID does not exist, this condition is not met.

  • Security Parameters (Password, TTL, and the like).

If the Open messages do not have any errors, the Hold Time is negotiated (using the lower value), and a KEEPALIVE message is sent (assuming the value is not set to zero). The connection state is then moved to OpenConfirm. If an error is found in the OPEN message, a Notification message is sent, and the state is moved back to Idle.

If TCP receives a disconnect message, BGP closes the connection, resets the ConnectRetryTimer, and sets the state to Active. Any other input in this process results in the state moving to Idle.


In this state, BGP waits for a Keepalive or Notification message. Upon receipt of a neighbor’s Keepalive, the state is moved to Established. If the hold timer expires, a stop event occurs, or a Notification message is received, and the state is moved to Idle.


In this state, the BGP session is established. BGP neighbors exchange routes via Update messages. As Update and Keepalive messages are received, the Hold Timer is reset. If the Hold Timer expires, an error is detected and BGP moves the neighbor back to the Idle state.

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