e learned from two routing pro- tocols within a single router, the protocol with the lowest administrative distance will win. The administrative distance is the value assigned to each routing protocol to allow the router to prioritize routes learned from multiple sources. The administrative dis- tances for the various routing protocols are shown in Table 9-1. Table 9-1. Routing protocols and their administrative distances Route type Administrative distance Connected interface Static route Enhanced Interior Gateway Routing Protocol (EIGRP) summary route External Border Gateway Protocol (BGP) Internal EIGRP Interior Gateway Routing Protocol (IGRP) Open Shortest Path First (OSPF) Intermediate System-Intermediate System (IS-IS) Routing Information Protocol (RIP) Exterior Gateway Protocol (EGP) On Demand Routing (ODR) External EIGRP Internal BGP Unknown Spanning tree, discussed in Chapter 8, isn't really a routing protocol, because the protocol doesn't care about the data being passed; spanning tree is only concerned with loops and with preventing them from a physical and Layer-2 perspective. In other words, spanning tree is con- cerned more with determining that all possible paths within its domain are loop-free than with determining the paths along which data should be sent. Routing Tables When a packet arrives at a router, the router determines whether the packet needs to be forwarded to another network. If it does, the router checks the RIB to see whether it contains a route to the destination network. If there is a match, the packet is adjusted and forwarded out the proper interface to where it belongs (see Chapter 15 for more information on this process). If no match is found in the RIB, the packet is forwarded to the default gateway, if one exists. If no default gateway exists, the packet is dropped. Originally, the destination network was described by a network address and a subnet mask. Today, destination networks are often described by a network address and a prefix length. The network address is an IP addre