What is AD Value in Networking?

Introduction

One of the most important concepts in routing is administrative distance (AD). AD is a metric that determines the reliability or trustworthiness of a routing source. With the help of AD value, routers can easily choose the best route among multiple routes to the same destination. In this blog, we will explain the concept of AD or Administrative Distance in networking. We will discuss the different purposes of AD, how it operates, and its advantages and disadvantages.

Before getting into more details, let’s first understand what AD value really is.

What is AD Value in Networking?

AD represents a value assigned by routers to each route source. The lower the number, the higher the reliability or preference of that route source becomes. For instance, a route obtained from a connected interface possesses an AD value of 0 indicating it as the trusted option. On the other hand, a route learned through a static route carries an AD value of 1 signifying its considerable reliability but not as much as a directly connected route.

If we consider a routing protocol like OSPF routes acquired through it have an AD value of 110. This value signifies that they are less reliable than a static route but more dependable than other dynamic routing protocols available.

Below we have shown the Ad value and how the priority is chosen.

AD is also referred to as route preference or route ranking; however, it is only utilized within routers. It is not shared with other routers in the network. Different router vendors usually have their predefined default values for AD associated with types of route sources.

For instance, when it comes to Cisco routers the default values that are used are as follows:

Note: It is possible to modify the administrative distance value of a route using configuration commands. This can be helpful in situations where you want to influence the process of selecting routes based on your network design and preferences.

For better understanding, we have explained the purpose of AD value below.

Purpose of AD Value

The purpose of AD is to assist the router in selecting the optimal route, that is among multiple routes leading to the same destination. This situation can arise when the router is utilizing routing protocols or has sources providing routes for the same network. For better understanding, let’s consider a scenario where a router receives information about network 10.0.0.0/24 from three sources; a connected interface, a static route, and OSPF.

The router will compare the AD values associated with these sources. Once it is done with comparing the values, it chooses the one with the value. In this case, since the connected interface has an AD of 0 which is the lowest it becomes the chosen route while disregarding others.

AD also plays a role in preventing routing loops and ensuring routing operations. A routing loop occurs when packets are endlessly forwarded between routers without reaching their intended destination. This commonly happens due to inconsistent or incorrect routing information within the network. let’s consider two routers named R1 and R2 running OSPF and RIP for network 10.0.0.0/24 respectively. R1 receives information about this network from OSPF with an AD of 110.

After that, it advertises to R2 via RIP with an AD of 120. R2 learns about this network from RIP with an AD of 120. After that, it advertises back to R1 via OSPF with an AD value of 110. This results in a situation where R1 and R2 continuously exchange packets with each other for the network 10.0.0.0/24.

To avoid this, routers utilize administrative distance (AD) to filter out routes that are considered more reliable than their own.

How does AD works?

AD operates by comparing the values of sources for different routes leading to the destination network. The router follows these steps to determine the route;

• First, the router checks if there is a connected route for the destination network. If one exists it utilizes this route and afterwards terminates the process.
• If there is no connected route the router then examines whether there is a static route defined for the destination network. If so, it selects this route and after that, it stops the process.
• In cases where no static routes exist, the router proceeds to assess routes originating from routing protocols for the given destination network. If there are, the router will compare their AD values and select the route with the lowest value.
• In case there are routes with the same AD value the router will utilize its vendor-specific algorithm to resolve the tie and choose a single route.

Now that we have a better understanding of how AD works, let’s move on to the advantages and disadvantages of AD.

Advantages of Administrative Distance

Some of the advantages of AD are:

• It grants network administrators the ability to prioritize one routing protocol or static route over another.
• It offers flexibility and scalability, in designing and implementing networks.
• It improves network performance and reliability by preventing routing loops and selecting routes.
• It facilitates interoperability and integration of routing protocols and technologies.

Disadvantages of AD

Some of the disadvantages of AD are:

• Network troubleshooting and maintenance can become complex and confusing when using AD.
• If not configured consistently across routers it may lead to routing decisions or conflicts.
• Improper security measures or manipulation by actors can introduce security risks.

Frequently Asked Questions

Q1. What is ad value in OSPF?

OSPF has an AD value of 110 which gives it priority, over routing protocols like RIP or EIGRP.

Q2. What is the value of ad in protocol?

The value of the ad ranges from 0 to 255 and it is different for different protocols.

Q3. What is ad value in RIP protocol?

RIP protocol operates as a distance vector routing protocol using hop count as a metric for routing decisions. It functions at the network layer of the OSI model. Possesses an AD value of 120—making it less reliable than some alternative routing protocols.

Q4. What is the range of ad value?

The administrative distance values span from 0 to 255, on a scale where 0 signifies connectivity and 255 indicates a route.

Conclusion

In this blog, we have explained the administrative distance or AD value in networking. We have explored the purpose, and working of AD along with the different advantages and disadvantages associated with it.