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Understanding the Network: A Practical Guide to Internetworking by Martin J. Michael

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Route Selection

The router will use all the available sources of reachability information to construct the most accurate and efficient routing table possible, based on the available information. Each information source is assigned an administrative distance, which is used to determine a route's integrity. Table 10.2 lists the IOS's default administrative distance values for its supported IP routing protocols.

Table 10.2. IOS Administrative Distances
Protocol Distance
Connected interface0
Static route1
EIGRP summary route5
BGP (external)20
EIGRP (internal)90
IGRP100
OSPF110
RIP120
EIGRP (external)170
BGP (internal)200
Unknown255

The lower the administrative distance value, the more trusted the route. So, you can see in situations where multiple routing ...

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