Administrative Distance

In a routing table, each destination network (identified by its prefix and prefix length) can appear only once. Yet, it’s common for a router to learn about the same destination from multiple routing sources.

Administrative Distance (AD) is the value that helps the router decide which of those sources is more trustworthy. It assigns a reliability score to each route, allowing the router to prefer one source over another when multiple options exist.

Figure 1 – Router learns paths to the same network via OSPF and RIP

To see why this matters, let’s consider a practical example. In the network below, we have four routers, and PC1 needs to send traffic to a server in the 192.168.2.0/24 network.

When the traffic reaches the first router, R1 discovers two possible paths to reach 192.168.2.0/24:

• One path learned through the OSPF dynamic routing protocol

• Another path learned through the RIP dynamic routing protocol

Since OSPF and RIP each use their own metric to select the best path, this raises an important question:

• Which path should be installed in the routing table?

• Which routing source should R1 trust?

Figure 2 – Router faces a choice between two different network paths

This is exactly where Administrative Distance comes into play. In our example, it allows the router to make a choice when different routing sources point to the same destination.

The AD is a number assigned to each route that indicates how reliable the source is. The lower the value, the more the router will trust that route.

Administrative Distance is used first to select the most trusted routing protocol when multiple protocols advertise the same destination. Once the protocol is chosen, its metric is used to determine the best path within that protocol.

Below is a table showing the default Administrative Distance values for different types of routes.

Figure 3 - Default Administrative Distance

The smaller the Administrative Distance, the more trusted the route is. At the very top, we have directly connected routes with an AD of 0. This makes sense because these networks are physically connected to the router’s interfaces, there’s no need to question their reliability.

Next, we have static routes with an AD of 1. These are configured manually by a network administrator, which means they are considered highly reliable.

Then we move on to dynamic routing protocols. Here, I’ve listed the most common ones and the values you should know for the CCNA.

For example: