An autonomous system (AS), also known as a domain, is a collection of routers that are under a common administration, such as a company’s internal network or an Internet service provider’s (ISP’s) network. Because the Internet is based on the AS concept, two types of routing protocols are required:

■ Interior gateway protocols (IGP) are intra-AS (inside an AS) routing protocols. Examples of IGPs include Routing Information Protocol (RIP) version 1 (RIPv1), RIP version 2 (RIPv2), Open Shortest Path First (OSPF), Integrated Intermediate System-to-Intermediate System (IS-IS), and Enhanced Interior Gateway Routing Protocol (EIGRP).

■ Exterior gateway protocols (EGP) are inter-AS (between autonomous systems) routing protocols. Border Gateway Protocol (BGP) is the only widely used EGP protocol on the Internet. BGP version 4 (BGP-4) is considered the acceptable version of BGP on the Internet.

Different types of protocols are required for the following reasons:

■ Inter-AS connections require more options for manual selection of routing characteristics. EGPs should be able to implement various policies.

■ The speed of convergence (distribution of routing information) and finding the best path to the destination are crucial for intra-AS routing protocols.