s an autonomous system. The IGP running in each autonomous system is irrelevant to the external gateway protocol. The EGP knows only that a certain net- work 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. Chapter 10: Routing Protocols Figure 10-8. Interconnected autonomous systems The important thing to remember with external gateway protocols is that they really don't care how many routers there are, or what the speeds of the links are. The only thing an external gateway protocol cares about is traversing the least possible number of autonomous systems in order to arrive at a destination. 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 net- work 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 advertise- ments. All modern protocols are classless. EIGRP and OSPF are classless routing protocols. Poison reverse If a router needs to tell another router that a network is no longer viable, one of the methods employed is route poisoning. Consider RIPv1 as an example. Recall that a metric of 16 is considered unreachable. A router can send an update regarding a network with a metric of 16, thereby poisoning the entry in the rout- ing table of the receiving router. When a route