When building a BGP answer will experience it states:
Idle (Idle): router searches its routing table to see if there is a route to neighbors
Connect (connection): Router Discovery three neighbors and is ready to go the route TCP handshake. The successful establishment into the open sent, otherwise, becomes active (activated) waiting to re-establish a TCP connection.
Open sent (send): sends an OPEN message, and then wait for the arrival of an OPEN
Open confirm (acknowledgment): a router receives OPEN, check BGP session parameters, the matching confirmation sending keep alive, and then proceeds to wait for their OPEN insulation was confirmed.
Established (established): confirmation received keep alive their OPEN insulation, peer establishment; prepare the transfer route
If you receive a Notification message, then their OPEN not recognized, disconnect the TCP connection, move to the Idle state.
Active: change the normal state does not occur only when the parameters do not match, such as TCP establish unsuccessful, BGP and TCP connection initialization process will attempt to re-establish the neighbors will enter the state.
Neighbors can not be established in the case:
TCP is not successfully created (
ACL
IP unreachable)
Parameter mismatch causes the neighbors can not be established (
1 ipv4 to ipv6
）
Does not match the number AS, can not establish neighbor
BGP message defines the following types:
Open
Comprising a hold time and BGP router ID
Keepalive
Update
Send How to reach the purpose of network routing updates
And a network attributes include path
Notification
An error is detected
BGP closed connection to send a message
Route-refresh message