In this topic i will describe how BGP enabled routers are going to become neighbor in detail. As BGP does not use multicast we have to manually specify neighbors by using command
R1(config)# router bgp 100
R1(config-if)# neighbor 192.168.1.2 remote-as 200
R2(config)# router bgp 200
R2(config-if)# neighbor 192.168.1.1 remote-as 100
--> When ever we give the neighbor command, It will send TCP Connection request to the IP address which is mentioned in the command. Here R1 will send TCP Connection Request like this
Source IP = 192.168.1.1 ( Exit Interface IP address)
Destination IP = 192.168.1.2 ( IP address mentioned in Neighbor Command)
Port Number = 179
--> R2 will receive the request it will compare Source IP address with the IP address mentioned in the Neighbor Command, If both are matching TCP connection will be successful otherwise no TCP Connection ( This State IDLE - Active)
--> After this R1 will send Open Message to R2 Router with the parameters like AS number, Version and Hold time and it will receive Open Message from R2 router ( Open Sent)
--> Once It receives Open Sent Message it will wait for KeepAlive message from the neighbor ( Open Confirm).
--> Then they will exchange updates ( Established).
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