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 autono- mous 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 running in each autonomous system is unrelated to the external gateway protocol. The EGP knows only that a certain network is owned by a certain autonomous system. Let's say that 1.0.0.0/8 is within ASN 1, 2.0.0.0/8 is within ASN 2, 3.0.0.0/8 is within ASN 3, and SO on. For a device in ASN 1 to get to the network 10.0.0.0/8, the path might be through autonomous systems 1, 2, 3, 9, and 10. It might also be through autonomous systems 1, 2, 7, 8, 9, and 10, or even 1, 2, 7, 8, 3, 9, and 10. As with a distance-vector IGP counting hops, the fewer the number of autonomous systems traversed, the more appealing the route. The important thing to remember with EGPs is that they really don't care how many routers there are or what the speeds of the links are. The only thing an EGP cares about is traversing the least possible number of autonomous systems to arrive at a destination. Specific Routing Protocols Figure 10-8. Interconnected autonomous systems Before we go any further, let's define some key routing terms: Classful routing protocol A classful routing protocol is one that has no provision to support subnets. The natural state of the network is always advertised. For example, the network 10.0.0.0 will always be advertised with a subnet mask of 255.0.0.0 (/8), regardless of what subnet mask is actually in use. RIPv1 and IGRP are classful routing protocols. Classless routing protocol A classless routing protocol is one that includes subnet masks in its advertisements. All modern protocols a