15.5.3 Interior And Exterior Routing

Not all routing occurs within the same administrative network.

Large organizations, universities, and Internet Service Providers (ISPs) each manage their own networks independently. Such networks are known as Autonomous Systems (ASs).

Routing within an Autonomous System is called interior routing and is normally performed using Interior Gateway Protocols (IGPs). One of the most widely used IGPs is the Open Shortest Path First (OSPF) protocol, which enables routers within an organization to exchange routing information efficiently.

Communication between different Autonomous Systems requires exterior routing. The protocol used almost universally for this purpose is the Border Gateway Protocol (BGP). Rather than simply selecting the shortest path, BGP allows organizations to exchange routing information while applying administrative policies that reflect commercial agreements, security requirements, and network management objectives.

Consequently, OSPF primarily determines how packets move within an organization, whereas BGP determines how they move between organizations across the global Internet.