State machine in BGP

BGP State machine

Idle The initial state

BGP The agreement was initially in Idle state , Start to prepare TCP Connect to and monitor remote peers . In this state , The system does not allocate any resources , And refuse all those who enter BGP Connect . Only received Start Event when , Only then allocates BGP resources , start-up ConnectRetry timer , Start on other BGP Transport layer connection of peers , It also listens for connection requests from other peers .Connect Retry The timer will increase in series

Connect Connection status

In this state BGP wait for TCP Complete the connection , Then decide the follow-up action ：

1. If the connection is successful , Local emptying ConnectRetry timer , And send OPEN message , And then the state changes to OpenSent state ;
2. The connection fails ,BGP Will continue to listen for connections from neighbors , Reset Connect Retry Timer and transfer your state to Active state
3. Connect Retry Time out , Reset ConnectRetry timer , Try to establish with your neighbors again TCP Connect ,BGP keep Connect state , When other events occur, turn Enter into Idle state .

Active active

Underway TCP Connect , Waiting for completion , authentication It's all in TCP Completed during establishment .

1. If TCP Successful connection ,BGP take Connect Retry The timer is cleared , Initialization complete , Send to neighbors Open Message and set the status to Open,hold Time is set to 4mins .
2. If in Active state ,Connect Retry The timer times out and returns Connect Status and reset Connect Retry timer .
3. If you try with an unknown IP establish TCP conversation , meanwhile Connect Retry Timer reset , Reject the connection and keep Active state .
4. Any event will lead to a return Idle

OpenSent Parameter negotiation status

In this state ,TCP The connection has been established successfully , Start sending to neighbors Open Message packet , meanwhile BGP Waiting for the neighbor to send open Message packet ;Open The packet carries parameters to negotiate the establishment of peers .

1. received open news , If something goes wrong , Will send a notification Message and set the status to Idle .
2. If you receive open The package message is right , Will send keeplive Give it to your neighbor , And establish IBGP perhaps EBGP The state is set to Open confire state .
3. If you receive TCP Disconnect the message and disconnect BGP Connection reset Connect Retry timer , The state is set to Active .

OpenConfirm Parameter validation status

In this state , Parameters 、 Negotiation of capability characteristics succeeded , Send it yourself Keepalive package ,BGP wait for KEEPALIVE or NOTIFICATION message . If you receive KEEPALIVE message , entering Established state , If you receive NOTIFICATION message , It becomes Idle state .

1. If you receive one keeplive Message packet , Will set the status to Establish .
2. If you receive notification Message packet , Will set the status to Idle And disconnect TCP Connect .
3. If hold The timer timed out , An error is detected or stop events ,BGP Will send a notification And disconnect , Then the state is set to Idle .

Established Build success

In this state , ** Have received the other party's Keepalive package , The capability characteristics of both parties are found to be consistent through negotiation ,**BGP You can exchange with other peers UPDATE,NOTIFICATION,KEEPALIVE message .

1. If you receive the correct UPDATE or KEEPALIVE message , It is considered that the opposite end is in normal operation , Local reset Hold Timer.
2. If you receive NOTIFICATION message , Go locally to Idle state .
3. If you receive the wrong UPDATE message , Local delivery NOTIFICATION Message notification to peer , And change the local state to Idle.
4. If received TCP Chain removal notice , Local shutdown BGP Connect , And back to Idle state .