An autonomous system (AS) is a collection of routers under a common administration such as a company or an organization. An AS is also known as a routing domain. Typical examples of an AS are a company’s internal network and an ISP’s network.

The Internet is based on the AS concept; therefore, two types of routing protocols are required:

Note: Because BGP is the only EGP available, the term EGP is rarely used; instead, most engineers simply refer to BGP.

The example in the figure provides simple scenarios highlighting the deployment of IGPs, BGP, and static routing:

Note: BGP is beyond the scope of this course and is not discussed in detail.