ernal. Internal routes are usually considered to be more reliable than external routes, by a means called administrative distance. Excep- tions include BGP, which prefers external routes over internal ones, and OSPF, which does not assign different administrative distances to internal versus external routes. Communication Between Routers Metrics and Protocol Types The job of a routing protocol is to determine the best path to a destination network. The best route is chosen based on a protocol-specific set of rules. RIP uses the num- ber of hops (routers) between networks, whereas OSPF calculates the cost of a route based on the bandwidth of all the links in the network. EIGRP uses links' reported bandwidths and delays to determine the best path by default, and it can be config- ured to use a few more factors as well. Each of these protocols determines a value for each route. This value is usually called a metric. Routes with lower metrics are more desirable. Perhaps the simplest form of metric to understand is the one used by RIP: hop count. In RIP, the hop count is simply the number of routers between the router determining the path and the network to be reached. Let's consider an example. In Figure 10-4, there are two networks, labeled 10.0.0.0, and 20.0.0.0. Router A considers 20.0.0.0 to be available via two paths: one through Router B, and one through Router E. The path from Router A through Router B traverses Routers B, C, and D, resulting in a hop count of 3 for this path. The path from Router A through Router E traverses routers E, F, G, and D, resulting in a hop count of 4 for this path. Figure 10-4. Example of metrics in routing protocols Lower metrics always win, SO Router A will consider the path through Router B to be the better path. This router will be added to the routing table with a metric of 3. Using hop count as a metric has a limitation that can cause suboptimal paths to be chosen. Looking at Figure 10-5, you can see that the link between Routers B and C is a T1 running at 1.54 Mbps, while th