Inter-router communication

边界网关协议不使用你好这包发现rs 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 manually configured. BGP neighbors are defined by an IP address.

BGP uses TCP port 179 to communicate with other routers. 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.