Administrative Distance


We know that routers learn about adjacent networks that are directly connected and about remote networks by using static routes and dynamic routing protocols. In fact, a router might learn of a route to the same network from more than one source. For example, a static route might have been configured for the same network/subnet mask that was learned dynamically by a dynamic routing protocol, such as RIP. The router must choose which route to install.
Note: You might be wondering about equal cost paths. Multiple routes to the same network can only be installed when they come from the same routing source. For example, for equal cost routes to be installed they both must be static routes or they both must be RIP routes.
Although less common, more than one dynamic routing protocol can be deployed in the same network. In some situations it may be necessary to route the same network address using multiple routing protocols such as RIP and OSPF. Because different routing protocols use different metrics, RIP uses hop count and OSPF uses bandwidth, it is not possible to compare metrics to determine the best path.
So, how does a router determine which route to install in the routing table when it has learned about the same network from more than one routing source?

The Purpose of Administrative Distance
Administrative distance (AD) defines the preference of a routing source. Each routing source - including specific routing protocols, static routes, and even directly connected networks - is prioritized in order of most- to least-preferable using an administrative distance value. Cisco routers use the AD feature to select the best path when it learns about the same destination network from two or more different routing sources.
Administrative distance is an integer value from 0 to 255. The lower the value the more preferred the route source. An administrative distance of 0 is the most preferred. Only a directly connected network has an administrative distance of 0, which cannot be changed.
An administrative distance of 255 means the router will not believe the source of that route and it will not be installed in the routing table.
Note: The term trustworthiness is commonly used when defining administrative distance. The lower the administrative distance value the more trustworthy the route.
Click show ip route in the figure.
The AD value is the first value in the brackets for a routing table entry. Notice that R2 has a route to the 192.168.6.0/24 network with an AD value of 90.
D 192.168.6.0/24 [90/2172416] via 192.168.2.1, 00:00:24, Serial0/0/0
R2 is running both RIP and EIGRP routing protocols. (Remember: it is not common for routers to run multiple dynamic routing protocols, but is used here to demonstrate how administrative distance works.) R2 has learned of the 192.168.6.0/24 route from R1 through EIGRP updates and from R3 through RIP updates. RIP has an administrative distance of 120, but EIGRP has a lower administrative distance of 90. So, R2 adds the route learned using EIGRP to the routing table and forwards all packets for the 192.168.6.0/24 network to router R1.

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