BGP uses which underlying protocol to establish connection to peers?

BGP (Border Gateway Protocol) establishes connections to its peers using TCP as its underlying protocol, which operates over Internet Protocol version 4 (IPv4). TCP is defined by IP protocol number 6. This utilization of TCP is crucial because it provides reliable, ordered, and error-checked delivery of a stream of data, which is essential for the exchange of routing information between BGP speakers.

The other protocols mentioned in the options have different purposes: IP protocol 89 refers to the EGP (Exterior Gateway Protocol), IP protocol 17 is UDP (User Datagram Protocol), which is connectionless and does not ensure reliability, and IP protocol 47 is GRE (Generic Routing Encapsulation), a tunneling protocol that encapsulates a wide variety of protocols. None of these options provide the reliability and ordered transmission needed for a robust routing protocol like BGP. Consequently, the choice of IP protocol 6 is definitive for establishing BGP sessions.