What Is Administrative distance in Networking?

Think of administrative distance as the trust score a router gives to different sources of information about where networks are located. Imagine you are trying to find the best coffee shop in a new city. Your best friend, who lives there, tells you one place. A random person on the street tells you another. A billboard advertisement tells you a third. You are more likely to trust your best friend over the random person or the billboard. You assign a high trust level to your friend and a lower trust level to the other sources. That is exactly what a router does with administrative distance.

Every time a router learns a path to a network, it learns that path from a specific source. A network that is directly connected to one of its own interfaces is learned from the interface itself. A network that an administrator manually types in becomes a static route. A network learned by talking to other routers comes from a dynamic routing protocol like OSPF, EIGRP, or BGP. Each of these sources has a default trust score, which is the administrative distance.

When the router has two different paths to the same network, it cannot use both unless it is doing load balancing. It must pick one. It looks at the administrative distance of each path. The path with the lower number wins because the router trusts that source more. For example, a static route (distance 1) will always be chosen over an OSPF route (distance 110) to the same destination, even if the OSPF route is technically faster. Distance is about trust in the source, not about the speed of the path.

It is important to understand that administrative distance is only used when comparing routes from different routing protocols. If the same protocol learns two paths, they are compared using a different metric, like hop count in RIP or bandwidth in OSPF. Administrative distance is the first filter. It answers the question, Which source of information do I trust more? before the router even looks at how good the path is.

Administrative distance (AD) is a numerical value assigned to a routing information source. It is used by routers running multiple routing protocols to select the best route when the same destination prefix is learned via more than one protocol. The AD acts as a preference rating. Lower values indicate a more trustworthy or preferred source. The AD does not directly describe path quality such as latency, bandwidth, or hop count, but rather the reliability of the routing protocol itself as a source of routing information.

Every routing source has a default AD value defined by the router's operating system. Cisco IOS, for example, uses these well-known defaults: Connected interface routes have an AD of 0, making them the most trustworthy. Static routes pointing to a next-hop address have an AD of 1. EIGRP summary routes have an AD of 5. External BGP routes have an AD of 20. Internal EIGRP routes have an AD of 90. OSPF routes have an AD of 110. IS-IS routes have an AD of 115. RIP routes have an AD of 120. EIGRP external routes have an AD of 170. Internal BGP routes have an AD of 200. Unknown or unreliable sources have an AD of 255, meaning the route is never installed in the routing table.

When a router learns a route, it stores it in the routing information base (RIB). If multiple protocols provide a route to the same destination, the RIB installation process compares the AD values. The route with the lowest AD is installed in the routing table. The other routes are kept in the RIB as backup or alternative routes. If the installed route fails, the router can use the next-best candidate from the RIB. This process is fundamental to route selection in a multi-protocol environment.

Network administrators can manually adjust the AD of a specific route or a whole protocol. This is common when an administrator wants to influence route selection without disabling a protocol. For example, an administrator might increase the AD of a static route to 110 so that OSPF-learned routes are preferred over it. This is called floating static routing. The static route still exists but is only used if the OSPF route disappears.

Administrative distance is not the same as a routing metric. The metric is used by a single routing protocol to choose between multiple paths learned by that same protocol. For example, OSPF uses cost as its metric. If OSPF learns two paths to the same network, it compares the cost of each path and chooses the lower cost. AD is not involved in that decision because both paths come from the same source. AD only comes into play when the paths come from different sources, such as OSPF and RIP.

In addition to default values, AD is critical in redistribution scenarios. When routes are redistributed from one protocol into another, the redistributed routes inherit the AD of the receiving protocol. This can cause unexpected routing behavior if not carefully planned. Understanding and configuring AD is a core skill for network engineers working with complex routing environments.

Administrative distance matters because in real-world networks, routers rarely run just one routing protocol. A company might have OSPF for internal routing and BGP for connecting to the internet. A static route might be used for a small branch office. When these protocols all advertise the same network, the router needs a clear and consistent rule to decide which route to use. Without administrative distance, the router would have no way to compare routes from different protocols, and routing decisions would be unpredictable.

For network engineers, understanding AD is essential for designing redundant and reliable networks. You can intentionally set up floating static routes as backups. If the primary dynamic route fails, the static route takes over. This ensures connectivity even when the primary routing protocol goes down. If you do not understand AD, you might accidentally create a situation where a less reliable route is used all the time, or where a backup route never activates because the AD values are not configured correctly.

Another practical reason AD matters is troubleshooting. When a router is not sending traffic where you expect, the first thing to check is the routing table. If you see a route with a lower AD than you intended, that explains the behavior. For example, if you add a static route but it is not being used, it might be because OSPF has a lower AD. You need to verify the AD values and adjust them if needed.

AD also affects how traffic flows in complex networks with route redistribution. If you redistribute OSPF routes into EIGRP, the redistributed routes get the AD of EIGRP. If there are overlapping routes from another protocol, the AD values determine which route wins. Misconfigured AD values can cause routing loops or suboptimal routing. Professionals must master AD to maintain stable and predictable network behavior.

On the job, knowing AD helps you make intentional design decisions. You can choose which protocol the router should prefer without having to turn off a protocol entirely. This gives you flexibility and control. It also helps you explain why traffic takes a certain path to your colleagues or clients. In short, AD is a fundamental building block of route selection that every networking professional must understand.

Administrative distance questions on IT certification exams typically fall into three categories: recall of default values, route selection scenarios, and troubleshooting configurations. In recall questions, you are asked a direct question such as, What is the default administrative distance of OSPF? The choices are numbers like 90, 110, 120, and 170. You need to know that OSPF is 110. Another common recall question is, Which protocol has the lowest default administrative distance? with options like RIP, EIGRP, OSPF, and directly connected. The correct answer is directly connected because its AD is 0.

In route selection scenarios, you are given a router that learns the same network from two different protocols. For example, a router learns 192.168.1.0/24 via RIP (AD 120) and via a static route (AD 1). The question asks, Which route will be installed in the routing table? The answer is the static route because it has the lower AD. Sometimes the question adds a metric to confuse you. For instance, the RIP route has a metric of 2 hops, and the static route has a metric of 0. Even though the static route metric is better, the question is really about AD. You must ignore the metric and focus on AD first.

Troubleshooting questions present a problem. A network administrator configured a floating static route with an AD of 130 as a backup for an OSPF route (AD 110). The OSPF route fails, but the router still does not use the static route. The question asks why. The answer is that the static route has an AD of 130, but RIP (AD 120) might also have a route, or the floating static route might have been configured incorrectly. Alternatively, the static route might have an AD higher than other dynamic routes. You need to recognize that the AD must be higher than the primary protocol but lower than any other backup protocol for it to work as intended.

Another question pattern involves redistribution. A router redistributes OSPF routes into EIGRP. The redistributed routes now have an AD of 170 (EIGRP external). The original OSPF routes had an AD of 110. The question asks which route will be preferred when both are present. The OSPF route with AD 110 wins because it has a lower AD, even though it is the same network. This highlights how AD affects route selection after redistribution.

You might also see questions that require you to order routes by preference. For example, list the following routes from most preferred to least preferred: OSPF, static, directly connected, RIP. The correct order is directly connected (AD 0), static (AD 1), OSPF (AD 110), RIP (AD 120). These ordering questions test your knowledge of the default AD hierarchy.

Finally, some questions test the concept of floating static routes. You are asked, An administrator wants a static route to be used only if OSPF fails. What AD value should be assigned to the static route? The correct answer is any value greater than 110 and less than 255, typically 130 or 150. This tests your understanding of how AD controls route priority.

In a production network, administrative distance is one of the most important concepts to master because it directly influences how traffic flows. Network engineers often run multiple routing protocols for different purposes. For example, OSPF is used inside an enterprise, BGP is used for internet connectivity, and static routes are used for small remote sites. Without a clear understanding of AD, you can easily create routing loops or suboptimal traffic paths.

Let us look at a common configuration. An engineer configures OSPF on all core routers. They also need a default route to the internet. They configure a static default route on the border router pointing to the ISP. That static route has an AD of 1. They also configure OSPF to redistribute the default route. The redistributed default route in OSPF has an AD of 110. All internal routers learn two default routes: one from OSPF (AD 110) and one from the static route (AD 1). Because the static route has a lower AD, the internal routers will use the static route directly if they have it, or they will use the OSPF route if the static route is not present. This can cause traffic to bypass the intended path and might break load balancing.

To fix this, the engineer might set the AD of the static route to 115, so that the OSPF route (AD 110) is preferred. Or they might use a policy to control which route is advertised. This kind of tuning is common in real networks.

Another practical point is that you can change the AD of an entire protocol. On a Cisco router, you can use the command distance ospf intra-area 120 to change the AD of OSPF intra-area routes from 110 to 120. This is useful when you want to prefer EIGRP (AD 90) over OSPF for internal routes, but you still want to run OSPF for interoperability.

What can go wrong? The most common issue is that someone configures a floating static route with an AD that is too low. For example, they want a backup static route for OSPF, so they set the static route AD to 105. But OSPF's AD is 110. The static route now has a lower AD (105) and becomes the primary route, not the backup. This defeats the purpose of the backup. Always ensure that the floating static route's AD is higher than the primary protocol's AD.

Also, when troubleshooting, always check the routing table with show ip route. Look at the first number in brackets. For example, a route might appear as S 10.0.0.0/8 [1/0] via 192.168.1.1. The [1/0] means AD 1 and metric 0. If you see a route that you did not expect, check its AD. It could be that a static route is overriding a dynamic route, or vice versa.

Finally, remember that AD is configurable per route on some platforms. You can use the ip route command with an AD value at the end. For example, ip route 10.0.0.0 255.0.0.0 192.168.1.1 130 creates a static route with AD 130. This is the floating static route technique. Practice this in a lab environment to see how the routing table changes when the primary route fails.

Understanding AD allows you to design networks that are both redundant and predictable. It is a fundamental tool in the network engineer's toolbox. Take the time to experiment with it, and you will avoid many common pitfalls.