to single devices, and routers and switches often operate on all seven layers of the OSI stack. In today's world, where every device seems to be capable of anything, when should you pick a router rather than a switch? Routers tend to be WAN-centric, while switches tend to be LAN-centric. If you're connecting T1s, you probably want a router. If you're connecting Ethernet, you probably want a switch. Routing Tables Routing is a fundamental process common to almost every network in use today. Still, many engineers don't understand how routing works. While the Cisco certification process should help you understand how to configure routing, in this section, I'll show you what you need to know about routing in the real world. I'll focus on the founda- tions, because that's what most engineers seem to be lacking-we spend a lot of time studying the latest technologies and sometimes forget the core principles on which everything else is based. In a Cisco router, the routing table is called the route information base (RIB). When you execute the command show ip route, the output you receive is a formatted view of the information in the RIB. Each routing protocol has its own table of information. For example, EIGRP has the topology table, and Open Shortest Path First (OSPF) has the OSPF database. Each protocol makes decisions about which routes will be held in its database. Routing pro- tocols use their own metrics to determine which route is best, and the metrics vary widely. The metric value is determined by the routing protocol from which the route Chapter 9: Routing and Routers was learned. Thus, the same link may have very different metrics depending on the protocol used. For example, the same path may be described with a metric of 2 in the Routing Information Protocol (RIP), 200 in OSPF, and 156160 in EIGRP. Routing protocols and metrics are covered in more detail in Chapter 10. If the same route is learned from two sources within a single routing protocol, the one with the best metric will win. Should the same route b