Final answer:
BGP is the routing protocol that is not used within an autonomous system.
Step-by-step explanation:
The routing protocol that is not used within an autonomous system is Border Gateway Protocol (BGP).
Autonomous System (AS) is a collection of routers under a single administrative domain. BGP is designed to route traffic between different autonomous systems and is used to exchange routing information between autonomous systems. It is an external routing protocol.
On the other hand, Intermediate System to Intermediate System (IS-IS), Open Shortest Path First (OSPF), and Routing Information Protocol (RIP) are all internal routing protocols used within an autonomous system.