Introduction to Administrative Distance
Administrative distance (AD) is a value used by routers to rank routes from different routing protocols. AD represents the trustworthiness of the source of the route. Lower AD values are more preferred. If a router has multiple routes to a destination from different sources, it will choose the route with the lowest AD.
Static Routes and Backup Routes
When a dynamic routing protocol is not used, static routes can be employed. Static routes are manually configured routes. To ensure a backup route, multiple static routes to the same destination can be configured with different AD values.
Configuring Static Routes with Administrative Distance
The primary route is configured with a lower AD value, making it the preferred route. The backup route is configured with a higher AD value. In the event of the primary route failure, the router will then use the backup route.
Example Configuration:
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ip route 192.168.1.0 255.255.255.0 10.0.0.1 1
ip route 192.168.1.0 255.255.255.0 10.0.0.2 10
In the above example, 192.168.1.0/24 is the destination network.
10.0.0.1 is the next-hop IP address for the primary route with an AD of 1.
10.0.0.2 is the next-hop IP address for the backup route with an AD of 10.
Verification:
After configuration, use the show ip route command to verify that the primary route is in use and the backup route is listed as a candidate for use if the primary route fails.
References:
CompTIA Network+ guide explains the concept of administrative distance and its use in static routing configuration (see page Ref 9 Basic Configuration Commands).