Open : Starts the Session
Keepalive : Whether Neighbor router is there or not??
Update : Network Reachability Changes
Notification : Something Bad happens to close the session.
BGP Connection States
There are 6 states of BGP during BGP connection establishment
1) IDLE State : Verifies the route to neighbor,Start the TCP connection with its configured peer.
2) Connect State : Waits for Successful Negotiation with peer,BGP won't spend much time here.
3) Active State : Attempting to connect with neighbor, If the neighbor cannot connect it will end up in this phase
4) Open State : After successful establishment of connection, The BGP sends a Open message to neighbor.
5) Open Confirm : The BGP FSM will open the OPEN MESSAGE it will check its validity, and sends keepalive messages.
6) Established state: In this state the BGP exchanges information with peers.
Router# show ip bgp neighbors 172.16.232.178
BGP neighbor is 172.16.232.178, remote AS 10, external link
Index 1, Offset 0, Mask 0x2
Inbound soft reconfiguration allowed
BGP version 4, remote router ID 172.16.232.178
BGP state = Established, table version = 27, up for 00:06:12
Last read 00:00:12, hold time is 180, keepalive interval is 60 seconds
Minimum time between advertisement runs is 30 seconds
Received 19 messages, 0 notifications, 0 in queue
Sent 17 messages, 0 notifications, 0 in queue
Inbound path policy configured
Route map for incoming advertisements is testing
Connections established 2; dropped 1
Connection state is ESTAB, I/O status: 1, unread input bytes: 0
Local host: 172.16.232.181, Local port: 11002
Foreign host: 172.16.232.178, Foreign port: 179
The above message is displayed after successful establishment of BGP session.
Neighbor Table: The connected BGP Neighbors.
BGP Table : A list of all BGP Routes
Routing Tables: A list of Best routes.
R1> show ip bgp
BGP table version is 5, local router ID is 220.127.116.11
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
r> 18.104.22.168/24 10.10.13.3 0 130 0 30 i
*> 22.214.171.124/24 10.10.13.3 0 125 0 30 i