Entire books can and have been written on each of the routing protocols discussed in this chapter. My goal is not to teach you everything you need to know about the protocols, but rather, to introduce them and show you what you need to know to get them operational. I'll also include some of the commands commonly used to troubleshoot these protocols.
Routing protocols are divided into types based on their purpose and how they operate. The major division between routing protocols is that of internal gateway protocols versus external gateway protocols.
An internal gateway protocol, or IGP, is designed to maintain routes within an autonomous system. An autonomous system is any group of devices controlled by a single entity. An example might be a company or a school, but the organization does not need to be that broad—an autonomous system could be a floor in a building or a department in a company. Examples of IGPs include RIP, EIGRP, and OSPF.
An external gateway protocol, or EGP, is designed to link autonomous systems together. The Internet is the prime example of a large-scale EGP implementation. The autonomous systems—groups of devices controlled by individual service providers, schools, companies, etc.—are each self-contained. They are controlled internally by IGPs, and are interconnected using an EGP (in the case of the Internet, BGP).
Figure 10-8 shows how different autonomous systems might be connected. Within each circle is an autonomous system. The IGP ...