Which of the Following Has the Least Default Administrative Distance?

Introduction

In the realm of networking, routing protocols play a pivotal role in determining how data packets travel from one device to another. For professionals preparing for certifications like Cisco’s CCNA or CompTIA Network+, understanding the concept of administrative distance is essential. The question, Which of the following has the least default administrative distance?” is a common one in networking exams, testing your knowledge of how routers prioritize routes. This comprehensive Exam Prep Study Guide, brought to you by DumpsQueen, delves into the intricacies of administrative distance, its significance in routing decisions, and how to master related exam questions. With a focus on clarity and depth, this guide will equip you with the knowledge needed to excel in your certification journey.

What is Administrative Distance?

Administrative distance (AD) is a value used by Cisco routers to determine the trustworthiness of a route learned from different sources. When a router receives multiple routes to the same destination, it must decide which route to use. This decision is based on the administrative distance, a numerical value assigned to each routing protocol or route source. The lower the AD, the more preferred the route.

For instance, if a router learns about a network via both OSPF (Open Shortest Path First) and RIP (Routing Information Protocol), it will prioritize the route with the lower administrative distance. This mechanism ensures that the router selects the most reliable path, optimizing network performance. Understanding the default AD values for various routing protocols is crucial for network administrators and certification candidates alike.

Why Administrative Distance Matters

Administrative distance is not just a theoretical concept; it has practical implications in network design and troubleshooting. In real-world scenarios, routers often receive routing information from multiple sources, such as static routes, dynamic routing protocols, or directly connected networks. Without a mechanism like administrative distance, routers would struggle to make consistent routing decisions.

For example, a directly connected network is inherently more reliable than a route learned through a dynamic protocol like RIP. By assigning a lower AD to directly connected networks, Cisco ensures that routers prioritize these routes. This prioritization enhances network stability and efficiency, making administrative distance a cornerstone of routing logic.

Default Administrative Distance Values

To answer the question, Which of the following has the least default administrative distance?” you need to know the default AD values for common routing protocols and route sources. Below is a detailed breakdown of these values, as used in Cisco routers:

Directly Connected Interface: AD = 0
A directly connected interface represents a network physically attached to the router. Since this is the most reliable source of routing information, it has the lowest possible AD of 0.
Static Route: AD = 1
Static routes are manually configured by network administrators. Because they are explicitly defined, they are highly trusted, earning an AD of 1.
External BGP (eBGP): AD = 20
External Border Gateway Protocol routes are learned from external autonomous systems. Their relatively low AD reflects BGP’s role in large-scale, inter-domain routing.
Internal EIGRP (Enhanced Interior Gateway Routing Protocol): AD = 90
EIGRP is a Cisco-proprietary protocol known for its efficiency. Internal EIGRP routes, learned within the same autonomous system, have an AD of 90.
OSPF (Open Shortest Path First): AD = 110
OSPF is a link-state routing protocol widely used in enterprise networks. Its default AD of 110 makes it less preferred than EIGRP but more trusted than RIP.
IS-IS (Intermediate System to Intermediate System): AD = 115
IS-IS is another link-state protocol, commonly used in service provider networks. Its AD of 115 places it just below OSPF in the hierarchy.
RIP (Routing Information Protocol): AD = 120
RIP is an older distance-vector protocol with a default AD of 120, reflecting its lower reliability compared to modern protocols.
External EIGRP: AD = 170
External EIGRP routes, learned from another autonomous system, have a higher AD of 170, indicating lower preference.
Internal BGP (iBGP): AD = 200
Internal BGP routes, used within the same autonomous system, have a high AD of 200, making them less preferred.
Unknown/Invalid Route: AD = 255
Routes with an AD of 255 are considered invalid and are never used by the router.

From this list, it’s clear that a directly connected interface has the least default administrative distance, with an AD of 0. This makes it the most preferred route source in Cisco routing logic.

How Routers Use Administrative Distance

When a router receives multiple routes to the same destination, it follows a systematic process to select the best path. First, it compares the administrative distances of the routes. The route with the lowest AD is added to the routing table. If multiple routes have the same AD (e.g., two OSPF routes), the router uses additional metrics, such as the protocol’s cost or metric value, to break the tie.

For example, consider a router that learns about the network 192.168.1.0/24 through three sources: a directly connected interface (AD = 0), an OSPF route (AD = 110), and a RIP route (AD = 120). The router will choose the directly connected interface because it has the lowest AD. This decision ensures that the router relies on the most trustworthy source of routing information.

Common Exam Scenarios and Questions

Networking certification exams, such as the Cisco CCNA, often include questions about administrative distance to test your understanding of routing behavior. These questions may present a scenario where a router learns routes from multiple sources and ask you to identify the preferred route based on AD. To prepare effectively, you need to memorize the default AD values and understand how they influence routing decisions.

DumpsQueen Exam Prep Study Guide resources are designed to help you master these concepts. By practicing with real-world scenarios and sample questions, you can build confidence and improve your exam performance. Below are a few sample multiple-choice questions (MCQs) to illustrate the type of questions you might encounter.

Configuring and Verifying Administrative Distance

In some cases, network administrators may need to modify the default administrative distance to influence routing behavior. For example, if you want a router to prefer an OSPF route over an EIGRP route, you can adjust the AD values using Cisco IOS commands. This is a common task in advanced networking scenarios and may appear in higher-level certification exams like the CCNP.

To change the administrative distance for a routing protocol, you can use the distance command in the router’s configuration mode. For instance, to set the AD for OSPF to 80, you would enter:

Router(config)# router ospf 1
Router(config-router)# distance 80

This command lowers the AD for OSPF routes, making them more preferred than internal EIGRP routes (AD = 90). After making such changes, you can verify the routing table using the show ip route command to ensure the desired routes are being used.

DumpsQueen Exam Prep Study Guide resources include hands-on labs and configuration examples to help you practice these tasks. By simulating real-world scenarios, you can gain the skills needed to manage administrative distance effectively.

Common Mistakes to Avoid

When studying administrative distance, candidates often make a few common mistakes that can lead to incorrect answers on exams. Here are some pitfalls to watch out for:

Confusing Administrative Distance with Metrics: Administrative distance determines the preference of a route source, while metrics (e.g., OSPF cost or RIP hop count) are used to compare routes within the same protocol. Don’t mix up these concepts.
Assuming All Protocols Are Equal: Each routing protocol has a unique AD based on its reliability. Memorizing the default values is critical to answering exam questions correctly.
Overlooking Directly Connected Interfaces: Because they have an AD of 0, directly connected interfaces are always preferred. This is a common trick” question in exams.
Ignoring Static Routes: Static routes (AD = 1) are highly trusted and often preferred over dynamic protocols. Be sure to consider them in routing scenarios.

By staying mindful of these pitfalls, you can approach exam questions with confidence. DumpsQueen Exam Prep Study Guide materials are designed to address these common mistakes, helping you avoid errors and maximize your score.

Preparing for Your Certification Exam

To excel in questions about administrative distance, you need a structured study plan that combines theoretical knowledge with practical application. Here are some tips to help you prepare:

Memorize AD Values: Create flashcards or a cheat sheet with the default administrative distance values for each route source. Review them regularly to ensure retention.
Practice with Scenarios: Use practice exams and labs to simulate routing scenarios. DumpsQueen Exam Prep Study Guide resources offer a wealth of practice questions and hands-on exercises.
Understand Routing Logic: Go beyond memorization by understanding how routers use AD to make decisions. This deeper insight will help you tackle complex exam questions.
Leverage Quality Resources: Trust reputable study materials like those provided by DumpsQueen. Our Exam Prep Study Guide resources are crafted by experts to align with the latest certification objectives.

By following these strategies, you can build a strong foundation in administrative distance and boost your chances of passing your certification exam.

Why Choose DumpsQueen?

At DumpsQueen, we are committed to helping networking professionals achieve their certification goals. Our Exam Prep Study Guide resources are designed to provide accurate, up-to-date, and comprehensive content that aligns with the objectives of major certifications like CCNA and CompTIA Network+. Whether you’re studying administrative distance or other networking topics, DumpsQueen offers practice questions, labs, and study guides to support your journey.

Visit the DumpsQueen to explore our full range of Exam Prep Study Guide materials. With our expertly crafted resources, you can study smarter, not harder, and take the next step in your networking career.

Conclusion

Administrative distance is a fundamental concept in networking that determines how routers prioritize routes from different sources. By understanding the default AD values and their implications, you can confidently answer exam questions like Which of the following has the least default administrative distance?” and apply this knowledge in real-world scenarios. With a directly connected interface holding the lowest AD of 0, it’s the most preferred route source in Cisco routing logic.

This Exam Prep Study Guide, brought to you by DumpsQueen, has explored the intricacies of administrative distance, from its role in routing decisions to its configuration in Cisco IOS. By leveraging DumpsQueen high-quality study materials, practicing with sample questions, and avoiding common pitfalls, you can master this topic and excel in your certification exam. Visit the DumpsQueen today to access our comprehensive Exam Prep Study Guide resources and take your networking career to new heights.

Free Sample Questions

Question 1: A router learns about the network 10.0.0.0/24 through a static route, OSPF, and RIP. Which route will the router prefer?
A) Static route
B) OSPF
C) RIP
D) None of the above

Answer: A) Static route

Question 2: Which of the following route sources has the least default administrative distance?
A) OSPF
B) Directly connected interface
C) EIGRP (Internal)
D) RIP

Answer: B) Directly connected interface

Question 3: A router learns about a network via both eBGP (AD = 20) and internal EIGRP (AD = 90). Which route will be added to the routing table?

A) eBGP
B) Internal EIGRP
C) Both routes
D) Neither route

Answer: A) eBGP

Question 4: What is the default administrative distance of a route learned via RIP?
A) 90
B) 110
C) 120
D) 170

Answer: C) 120