What Route Would Have the Lowest Administrative Distance in Networking

Introduction

In networking, the concept of "administrative distance" plays a vital role in routing decisions. When multiple routing protocols are available, each has an associated administrative distance (AD), which indicates the trustworthiness of the route it provides. The route with the lowest administrative distance is the one that the router will prioritize when deciding which path to use to forward packets to their destination.

In this article, we will explore the concept of administrative distance in-depth, how it impacts routing decisions, and the importance of understanding AD when configuring and troubleshooting routers. If you're new to networking or looking to brush up on your knowledge, this comprehensive guide on administrative distance and routing will provide valuable insights to help you make informed decisions for your network's efficiency.

What is Administrative Distance (AD)?

Administrative distance (AD) is a value used by routers to determine the trustworthiness of different routing protocols. In simple terms, it acts as a "rating" that helps routers choose the best path when multiple routing protocols provide different routes to the same destination. Each routing protocol is assigned a default administrative distance, which can range from 0 to 255. The lower the AD, the more trusted the protocol is.

For example:

Directly connected routes have an AD of 0.
Static routes have an AD of 1.
EIGRP (Enhanced Interior Gateway Routing Protocol) has an AD of 90.
OSPF (Open Shortest Path First) has an AD of 110.
RIP (Routing Information Protocol) has an AD of 120.

Routers will prefer the route with the lowest AD when multiple protocols are available. For instance, if you have both OSPF and RIP providing routes to the same destination, the router will select the OSPF route since it has a lower AD (110) compared to RIP's AD (120).

Why is Administrative Distance Important?

Understanding administrative distance is crucial for network administrators as it directly impacts the performance and reliability of a network. In situations where there are multiple routing protocols or redundant paths, AD helps determine the preferred route. This ensures that the network chooses the most reliable and efficient path for packet delivery.

By manipulating AD values, network engineers can influence the routing decisions, prioritize certain protocols, or make backup paths less favored. However, adjusting AD should be done carefully, as improper changes can lead to network instability or suboptimal routing behavior.

The Role of Routing Protocols in Administrative Distance

Various routing protocols used in modern networks have different default administrative distances. Let's look at some of the most common routing protocols and their associated AD values:

1. Directly Connected Routes (AD = 0)

A directly connected route refers to a network segment that is physically attached to a router. This type of route has the lowest administrative distance, set to 0. Directly connected routes are always preferred because they represent the most reliable and immediate path for traffic.

2. Static Routes (AD = 1)

Static routes are manually configured by network administrators. They are given an AD of 1, which is higher than directly connected routes but still very trusted. Static routes are often used for critical paths or backup routes.

3. EIGRP (AD = 90)

EIGRP, a hybrid routing protocol, is commonly used in large-scale enterprise networks. It is highly efficient, with an AD of 90. EIGRP can provide fast convergence and support multiple network types, making it a popular choice in dynamic environments.

4. OSPF (AD = 110)

OSPF is a link-state routing protocol used in large enterprise networks. With an AD of 110, OSPF is less trusted than EIGRP but more trusted than RIP. OSPF is often chosen for its ability to scale well in complex networks and its support for large IP networks.

5. RIP (AD = 120)

RIP is one of the oldest distance-vector routing protocols. It uses hop count as its metric and has an administrative distance of 120. While RIP is easy to configure, its limitations in scalability and convergence speed make it less favored in modern networks, especially in comparison to OSPF or EIGRP.

How Routers Choose the Best Route

Routers make routing decisions based on multiple factors, and administrative distance is one of the most important of these factors. When a router receives multiple routes to the same destination from different routing protocols, it will compare their AD values. The route with the lowest AD is selected and used for packet forwarding.

For example:

If a router learns of a destination through both a static route (AD = 1) and an OSPF route (AD = 110), it will choose the static route because of its lower administrative distance.
If the static route becomes unavailable, the router will automatically switch to the OSPF route without requiring manual intervention.

This automatic switching between routes is a key feature of how AD helps maintain network stability and performance.

Configuring Administrative Distance on Routers

In some cases, network administrators may want to influence routing decisions by adjusting the administrative distance. This can be done to prioritize certain routing protocols over others. For example, an administrator might want EIGRP to take precedence over OSPF, even though OSPF has a lower AD by default.

To modify the AD of a routing protocol, you can use the following commands (assuming you're using Cisco devices):

For EIGRP:

router eigrp [AS number]

distance [new AD] [destination network] [mask]
For OSPF:

router ospf [process ID]

distance [new AD] [destination network] [mask]

These commands allow you to change the AD value of specific protocols and destinations, giving you greater control over the routing behavior of your network.

Troubleshooting Administrative Distance Issues

Sometimes, the wrong route may be selected due to administrative distance issues. If you're experiencing unexpected routing behavior, you can troubleshoot by checking the following:

Verify the AD values of the routing protocols involved.
Check the routing table to ensure that the expected route with the lowest AD is being used.
Test failover by disabling one of the routing protocols and observing the route selection process.
Consider adjusting the AD values to force the router to prefer one protocol over another.

By understanding and managing administrative distance properly, you can ensure that your network routes traffic efficiently and reliably.

Conclusion

In summary, understanding administrative distance (AD) is crucial for network administrators in order to make informed decisions about routing and ensure that traffic is efficiently forwarded across a network. By understanding the various AD values for routing protocols, how routers choose the best route, and how to adjust AD when necessary, network professionals can optimize their routing configurations and troubleshoot issues more effectively.

The route with the lowest administrative distance is the one that will always be selected by the router, making it important to carefully configure and manage AD values for a stable and reliable network. Keep in mind that while directly connected routes have the lowest AD (0), other protocols like static routes, EIGRP, OSPF, and RIP are also used in various scenarios, each with their own default AD values.