Which routing protocol is primarily used for routing between autonomous systems?

The correct choice is BGP, which stands for Border Gateway Protocol. BGP is specifically designed for routing between autonomous systems (AS), which are large networks or groups of networks that are under the control of a single organization or entity.

BGP is classified as a path vector protocol, and it plays a crucial role in the internet, allowing different networks to communicate with each other. It manages how packets are routed across the internet by maintaining a table of network paths. BGP makes routing decisions based on paths, network policies, or rule sets configured by a network administrator. This capability is essential for the general structure and functioning of the internet, where different ASes need to exchange routing information reliably.

Other protocols like OSPF and IS-IS are designed for intra-domain routing, meaning they are meant for routing within a single autonomous system. They do not operate between different ASes, which is BGP's primary function. RIPv2 is another intra-domain protocol that focuses on smaller, simpler networks and does not scale as effectively as BGP for handling the complexities of inter-domain routing. Thus, BGP is the only protocol listed that is specifically used for routing between autonomous systems.