Administrative Distance (AD) Explained

A router can learn routing information from a variety of sources. If it learns routing information for the same destination from two or more sources, it compares the AD values of those sources to determine which source is more reliable. An AD value is the trustworthiness of the source. Routers assign an AD value to each source from the range 0 – 255. In this range, a smaller number is considered more reliable than a larger number. For example, a source with an AD value of 40 is more trustworthy than one with an AD value of 50. Routers assign the value 0 to the most reliable source and 255 to the least reliable source.

In simple terms, Administrative Distance (AD) is a scale that a router uses to measure the trustworthiness of a source that provides routing information.

The following table lists the default AD value of some of the most common routing sources.

If required, you can change the default AD values. You can change the default AD value of a particular routing protocol, a specific route, or even a static route. If you change the AD value of a source, the router will use the updated AD value when comparing it with other sources.

Basic concepts of Administrative Distance (AD)

A router can learn routing information from the following sources:-

• Interface configuration
• Manual configuration
• Routing protocols

Interface configuration

A router's interface, if connected to a LAN segment, serves as the default gateway for that segment. When you assign an IP configuration to an interface, the router automatically extracts the network information from the IP configuration and adds it to the routing table. For example, if you assign the IP address 10.0.0.1 255.0.0.0 to the F0/0 interface, the router adds the following entry to the routing table.

Network 10.0.0.0 255.0.0.0 exit interface F0/0

Manual configuration

You can manually add routing information to the routing table. The 'ip route' command in global configuration mode allows you to add a custom route.

Routing protocols

You can configure a routing protocol. The routing protocol learns and adds all routing information to the routing table. Depending on the network topology, multiple routes to the same destination can exist.

To select the best route for each subnet, a routing protocol uses the metric. A metric is the cost of a route. The routing protocol computes the metric for all routes and compares them to select the best route for each subnet. After choosing a single best route for each subnet, the routing protocol provides all routing information to the router. The router checks each route and adds the route to the routing table if it meets certain conditions.

Different routing protocols use distinct metrics to choose the best route. If only one routing protocol is running on a router, the router has no choice but to use the routes selected by that protocol. However, if multiple routing protocols are running on a router, the router may get different routes marked as the best route for the same destination from them. In such a situation, the router checks the AD value of each routing protocol. The router selects the routing information from the routing protocol with the lower AD value. For example, a router receives the routing information for the same destination from RIP and EIGRP. The AD values of RIP and EIGRP are 120 and 90, respectively. Since the AD value of EIGRP is lower than that of RIP, the router will select the routing information from EIGRP.

Administrative Distance (AD) Example

The following image shows a network.

In this network, Router0 has two routes to reach the network 200.0.0.0/24. The first route is via the S0/0/1 interface, and the second route is via the S0/0/0 interface. The route the router uses depends on the routing configuration. If you manually add the route to the routing table, the router will use it. If you configure a routing protocol, it will select the best route and add it to the routing table. The router will use the added route. If you use both options ( first, add a route manually, then configure a routing protocol), the router will use the manually added route. The default AD value (1) of a static route is less than that of all routing protocols.

Suppose you configured the RIP routing protocol. RIP uses the number of routers (hops) in the path to select the best route. It chooses the route with the fewest routers in the path. In this situation, Router0 will choose the second route (via S0/0/0) to reach the network 200.0.0.0/24. The following image shows the selection process.

To verify it, use the 'show ip route' command. The following image shows the output of this command.

Currently, only one routing protocol is running in the network. The router uses the route selected by the routing protocol. Suppose you configure another routing protocol without removing the current one, and both protocols choose different routes to the same destination. In that case, the router will use the route selected by the routing protocol with the lower AD.

Suppose you configure EIGRP in this network. By default, EIGRP uses the configured bandwidth and delay on all exit interfaces along the path to compute the route's metric. After computing the metric for all paths, it selects the path with the lowest metric.

The metric of the first route is 2684416. The metric of the second route is 256514560. Since the first route has a lower metric, EIGRP will select this route to reach the network 200.0.0.0/8. To verify this, you can use the 'show ip eigrp topology' command. The following image shows the output of this command.

Now, Router0 has two different routes reported as the best routes to reach the network 200.0.0.0/24.

• RIP says the second route (via - serial0/0/0) is the best route to reach the network 200.0.0.0/24.
• EIGRP says the first route (via – serial0/0/1) is the best route to reach the network 200.0.0.0/24.

In this situation, the router compares the AD values of both routing protocols and selects the route reported by the routing protocol with the lower AD value. The AD value for EIGRP is 90, and for RIP it is 120. Since the AD value of EIGRP is lower than that of RIP, the router will select the EIGRP route. To verify this, use the 'show ip route' command. The following image shows the output of this command.

Conclusion

Administrative Distance (AD) is a trustworthiness scale that routers use to determine the most reliable source of routing information. It uses values from 0 to 255. A lower value indicates higher trust. In a scenario where multiple routing methods are available, such as manually configured routes, static routes, or various dynamic routing protocols, AD helps routers decide which route to use for each destination. For example, a static route with an AD of 1 will always take precedence over routes learned from dynamic routing protocols such as RIP or EIGRP, because of its lower AD.