BGP CHAPTER 9 - BGP MESSAGES

Its is very important to have good understand of BGP Messages before Understanding BGP Neighborship Negotation and States (FSM).

BGP Message Type -

BGP Speaker (Router) uses below 4 messages to communicate , to form Neighborship and to exchange routing information with other BGP Speaker.

1)Open
2)Update 
3)Keepalive 
4)Notification

1)OPEN MESSAGE:-
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• As we are aware that BGP Uses TCP Port 179 and Before forming BGP Neighborship an TCP connection must be established.
• When TCP connection is established between two BGP Speakers an OPEN message is sent by BGP Speakers to each other.
• This OPEN message contains many parameters which plays an very important role while forming BGP Neighborship.   These parameters are -

a)Version
b)ASN
c)Hold Timer
d)BGP Identifer
e)Many Optional Parameters.

a)Version - 

• Must Match Between 2 BGP Speakers.
• Currently Version 4 is by default BGP Version.
• Incase the version doesnot match its gets negotiated to the lower value .For eg- If a BGP Speaker have BGP Version as 4 and another BGP Speaker have BGP Version 3 then the version will negotiated to Version 3.

b)Autonomous Sytem Number (ASN) -

• Determine BGP Neighborship is eBGP or iBGP.

c) Hold Timer -

• Hold Time is number of seconds the BGP Speaker allow between Receipt of Keepalive and/or Update Message .
• Within Hold time if no Message is received ; neighbor is considered as "DEAD".
• Must Match Between 2 BGP Speakers.
• Incase its differs , lower value will be selected .
• HOLD TIME = 3 X KEEPALIVE TIME.
• By default Hold Time is 180 Seconds.

d)BGP Identifer -

Its Highest Loopback address configured on the BGP Router(Speaker) and in absence of Loopback interface it will be the Highest IP address of an active Physical Interface on the BGP Router(Speaker)

2) UPDATE MESSAGE :-
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• This message contains the actual route updates
• It compose of below information-

a) NLRI -Network Layer Reachability Information-BGP Prefix Information - Prefixes which the BGP Speaker is advertising.
b) It includes Withdrawn Route Length , Withdrawn Route (incase when any route is removed)
c) Path Attributes - ORIGIN , AS PATH , NEXT HOP , MULTI_EXIT_DISC(MED)


3)KEEPALIVE MESSAGE:-
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• Once the BGP Neighorship is up and running ,these Keepalive messages are send continuously between BGP Speaker in a specific interval which is known as Keepalive Interval or Keepalive Time.
• Keepalive messages are send between BGP Speakers to verify whether peering BGP Neighbor is live or not .
• By default Keepalive Time is 60 Seconds. 

4)NOTIFICATION MESSAGE :-
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• Notification Messages  provides Notifications regarding BAD Messages / Error Messages
• These messages appears in case of misconfiguration or when BGP Peer goes down .

Hope you all find this information useful

Thanks for reading !!!