Introduction

In the world of dynamic routing protocols, Open Shortest Path First (OSPF) holds a prestigious position due to its scalability, robustness, and efficiency. As a link-state routing protocol, OSPF makes decisions based on link metrics, known as costs, rather than relying solely on hop counts like RIP (Routing Information Protocol). This blog post explores a common scenario involving OSPF cost calculations and addresses the query: “Refer to the exhibit. What is the OSPF cost to reach the Router A LAN 172.16.1.0/24 from B?” The objective of this comprehensive blog, brought to you by DumpsQueen, is to clarify how OSPF computes route costs, what affects those computations, and how one can determine the OSPF cost in practical scenarios. Whether you're preparing for Cisco certifications, understanding real-world enterprise network designs, or troubleshooting route advertisements, this deep dive will provide you with a strong foundation.

The Role of OSPF in Network Routing

OSPF is a dynamic, link-state routing protocol defined in RFC 2328. It operates using a Shortest Path First (SPF) algorithm developed by Edsger Dijkstra. Routers exchange link-state advertisements (LSAs) with neighboring routers tRouter(config-router)# auto-cost reference-bandwidth 10000 o construct a topological map of the network. Based on this map, each router independently calculates the shortest path to every network using SPF. OSPF doesn’t merely count hops like some older protocols. Instead, it calculates the cost of reaching a destination based on the bandwidth of the links between routers. The lower the cost, the more preferred the route. By default, OSPF cost is determined using the formula: Cost = Reference Bandwidth / Interface Bandwidth The reference bandwidth in OSPF is typically set to 100 Mbps (100,000,000 bits per second). However, this value can be manually adjusted depending on the needs of the network or to better reflect modern high-speed links.

Understanding the Scenario: From Router B to LAN 172.16.1.0/24

To accurately answer the question “What is the OSPF cost to reach the Router A LAN 172.16.1.0/24 from B?”, we must examine the components involved. Even though we are told to "refer to the exhibit", in professional and exam environments like the ones DumpsQueen prepares students for, the exhibit often includes a diagram or a routing table showing the path from Router B to Router A, including all intermediate routers and interface bandwidths.

Let’s create a hypothetical—but realistic—network topology to model this scenario:

Router A connects to LAN 172.16.1.0/24 via a FastEthernet interface (100 Mbps).
Router B connects to Router C via a Serial interface (1.544 Mbps).
Router C connects to Router A via a Gigabit Ethernet interface (1000 Mbps).

To compute the OSPF cost from B to 172.16.1.0/24, we must consider each hop and the interfaces used.

Calculating the Costs Step-by-Step

1. From Router B to Router C: Let’s assume this connection is over a T1 Serial link (bandwidth of 1.544 Mbps).

Using the OSPF formula: Cost = 100,000,000 / 1,544,000 ≈ 64

2. From Router C to Router A: This is via a Gigabit Ethernet link (1000 Mbps or 1,000,000,000 bits/sec).

Cost = 100,000,000 / 1,000,000,000 = 0.1 (rounded up to 1)

3. From Router A to LAN 172.16.1.0/24: The connection is a FastEthernet link (100 Mbps). Cost = 100,000,000 / 100,000,000 = 1 However, this cost is not included in the path cost as the destination is directly connected to Router A. Total OSPF Cost from Router B to 172.16.1.0/24: Router B to Router C: 6Router C to Router A:

Total OSPF cost = 64 + 1 = 65 This is the value that OSPF uses to determine the best route to the 172.16.1.0/24 LAN from Router B.

OSPF Cost Optimization in Enterprise Networks

In enterprise environments, OSPF cost optimization is crucial for achieving redundancy, load balancing, and fault tolerance. By default, OSPF will always select the lowest-cost path, even if multiple paths exist. Network administrators can manually modify OSPF interface costs using configuration commands to influence the routing decisions. For example: Router(config-if)# ip ospf cost <value> This command is often used to enforce policies like preferring certain links over others (e.g., fiber over copper) or balancing traffic loads.Another approach involves adjusting the reference bandwidth to accommodate higher-speed links. This ensures that Gigabit and 10-Gigabit interfaces do not end up with the same cost due to the limitations of the default 100 Mbps reference. This command sets the reference bandwidth to 10,000 Mbps (10 Gbps), providing a more granular cost distribution.

Common Issues with OSPF Cost Calculation

While OSPF’s cost model is straightforward, issues arise due to inconsistent configurations across routersDefault Reference Bandwidth: If not consistent across routers, costs may be calculated differently.Incorrect Bandwidth Settings: Misconfigured interface bandwidths can result in inaccurate cost calculations.Manually Configured Costs: While useful for control, manual cost overrides can introduce unexpected behavior if not well documented. These issues are critical in certification exams and real-world troubleshooting. At DumpsQueen, our training materials and practice exams ensure that you're not just memorizing values but understanding the concepts behind them.

The Importance of Exhibits in Network Exams

Many network certification exams from vendors like Cisco or CompTIA rely heavily on visual exhibits. The phrase “Refer to the exhibit” is common in questions that test not just theory but practical understanding. These exhibits can show routing tables, interface details, topology maps, or configuration snippets. When faced with such questions, follow these steps:Identify the path from source to destinatioNote the interface types and bandwidths.Calculate the OSPF cost for each hopSum the costs to determine the total path cost. This methodical approach is emphasized in all DumpsQueen practice exams to ensure test-takers can handle both simple and complex scenarios.

Free Sample Questions

To help you solidify your understanding, here are a few sample multiple-choice questions, similar to what you might find on real exams:

Q1: Refer to the exhibit. What is the OSPF cost to reach the Router A LAN 172.16.1.0/24 from Router B?

A. 65
B. 2
C. 1
D. 100

Correct Answer: A

Q2: Which OSPF command is used to manually set the cost on an interface?

A. ip route ospf cost
B. set ospf bandwidth
C. ip ospf cost
D. ospf manual-cost

Correct Answer: C

Q3: Which of the following formulas is used to calculate OSPF cost?

A. Bandwidth × 100
B. Reference Bandwidth / Interface Bandwidth
C. Interface Bandwidth – Load
D. Hop Count × Bandwidth

Correct Answer: B

Q4: What happens if the reference bandwidth is not consistent across all OSPF routers in a domain?

A. Routers will shut down OSPF automatically
B. OSPF will switch to RIP protocol
C. Each router will calculate different path costs
D. The OSPF topology will reset every 30 seconds

Correct Answer: C

Conclusion

OSPF is more than just a dynamic routing protocol—it's a powerful tool for controlling and optimizing network traffic in large-scale environments. Understanding how OSPF cost is calculated is essential not just for passing certification exams but also for designing and maintaining efficient networks. When presented with a question like “Refer to the exhibit. What is the OSPF cost to reach the Router A LAN 172.16.1.0/24 from B?”, knowing how to analyze the topology, interface types, and bandwidth settings allows you to arrive at the correct solution confidently. At DumpsQueen, we specialize in helping candidates master these concepts through detailed explanations, scenario-based practice questions, and updated dumps that reflect real exam environments. Whether you're aiming for your CCNA, CompTIA Network+, or any advanced routing certification, trust DumpsQueen to be your guide to success. Stay tuned for more deep dives into routing protocols, configuration best practices, and exam-focused content designed to elevate your networking career.