External EIGRP Internal BGP Unknown Administrative Distance A static route to a connected interface has an administrative distance of 0, and is the only route that will override a normal static route. A route sourced with an administrative distance of 255 is not trusted, and will not be inserted into the routing table. Looking at Table 10-1, you can see that RIP has an AD of 120, while OSPF has an AD of 110. This means that even though the RIP route has a better metric, in Figure 10-7, the route inserted into the routing table will be the one provided by OSPF. Specific Routing Protocols 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 proto- cols, 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 ver- sus 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 i