BGP States

Question 1

What are the BGP states?

Answer 1

• Idle
• Connect
• Active
• OpenSent
• OpenConfirm
• Established

Question 2

What 6 things happen in the IDLE state?

Answer 2

• Start event detected
• Initializes all BGP resources
• Starts ConnectRetry timer
• Initializes TCP connection to the neighbor
• Listens for TCP connection from neighbor
• Transitions to Connect state

Question 3

What 4 things happen in the CONNECT state?

Answer 3

• Waiting for TCP connection to complete
• Once 3WHS succeeds ConnectRetry timer cleared
• Sends Open message to neighbor
• Transitions to OpenSent state

Question 4

What 5 things happen in a successful ACTIVE state?

Answer 4

• BGP tries to initiate TCP connection to neighbor
• BGP clears the ConnectRetry timer when TCP connection suceeds
• BGP completes initialization
• Sends OPEN message to neighbor
• Transitions to OpenSent state

Question 5

• What 8 things happen in the OpenSent state?

Answer 5

• Open message has been sent to neighbor
• Waiting to hear Open message from neighbor
• When Open message is received all fields are checked
• Keepalive message is sent
• Keepalive timer is set
• Hold time is negotiated
• Peer connection determined to be iBGP or eBGP
• Transitions to OpenConfirm state

Question 6

What 2 things happen in the OpenConfirm state?

Answer 6

• BGP waits for a keepalive message
• Transitions to Established state when Keepalive message is received

Question 7

What 3 things happen in the Established state

Answer 7

• BGP connection is fully established
• BGP peers exchange Update and Keepalive messages
• Hold timer is reset when Update or Keepalive messages are received

Question 8

Connect State: what 3 things happen if the tcp connection initialized by the local host fails?

Answer 8

• Local host continues to wait for its peer to establish a TCP connection
• Then resets ConnectRetry timer
• Transitions to Active state

Question 9

Connect State: what 4 things happen if the ConnectRetry timer expires?

Answer 9

• Remains in Connect state
• Resets ConnectRetry timer
• Another TCP session is attempted
• Any other input event causes transition to IDLE

Question 10

Active State: what 3 things happen if the ConnectRetry timer expires while in Active State?

Answer 10

• BGP transitions back to Connect State
• Resets ConnectRetry timer
• TCP connection is tried again in Connect State

Question 11

Active State: what 4 things happen if while in Active State the neighbor attempts to connect from an unexpected IP address?

Answer 11

• ConnectRetry timer is reset
• Connection is refused
• Local process remains in Active State
• Any other input event transitions to IDLE

Question 12

OpenSent State: what 2 things happen if errors exist in the Open message received from the peer?

Answer 12

• Notification message sent
• State transitions to IDLE

Question 13

OpenSent State: what 4 things happen if a TCP disconnect is received?

Answer 13

• Local host closes the BGP connection
• Resets ConnectRetry timer
• Begins listening for TCP connection from peer
• Transitions to Active State

Question 14

OpenConfirm State: what 4 things happen if no Keep-alive is heard from the neighbor?

Answer 14

• Local host waits for Hold timer to expire
• Notification sent to neighbor
• BGP connection closed
• Transitions to IDLE