Inter-router communication

BGP does not use hello packets to discover neighbors like IGP protocols and cannot discover neighbors dynamically. BGP was designed as an inter-autonomous routing protocol, implying neighbor adjacencies will not change frequently and are coordinated. BGP neighbors are defined by an IP address.

BGP uses TCP port 179 to communicate with other routers. IGP protocols follow the physical topology because the sessions are formed with hellos that cannot cross network boundaries (single hop only). BGP uses TCP, which is capable of crossing network boundaries (multihop capable). While BGP can form neighbor adjacencies that are directly connected, it can also form adjacencies that are multiple hops away. Multihop sessions require that the router use an underlying route installed in RIB to establish the TCP session with a remote endpoint.

In the following scenario, R1 is able to establish a direct BGP session with R2. In addition, R2 is able to form a BGP session with R4, even though it passes through R3.