Multiple routing protocols and static routes can be used at the same time. If there are several
sources for routing information, an administrative distance value is used to rate the trustworthiness
of each routing information source. By specifying administrative distance values, Cisco IOS
Software can discriminate between sources of routing information.
An administrative distance is an integer from 0 to 255. A routing protocol with a lower
administrative distance is more trustworthy than one with a higher administrative distance. As
shown in Figure 3-4, if Router A receives a route to network 172.16.0.0 advertised by EIGRP and
by OSPF at the same time, Router A would use the administrative distance to determine that
EIGRP is more trustworthy.
102 Chapter 3: Medium-Sized Routed Network Construction
Figure 3-3 Routing Protocol Metrics
Figure 3-4 Administrative Distance
Router A would then add the EIGRP route to the routing table because EIGRP is a more trusted
routing source than OSPF. The administrative distance is an arbitrary value that Cisco IOS sets to
handle cases when multiple routing protocols send information about the same routes.
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