Troubleshooting Methodology: OSI Layer Checks Establish a Theory

N10-009 By islever Editorial Last updated

Explain the troubleshooting methodology. Explain concepts related to the Open Systems Interconnection (OSI) reference model.

Short answer

A After the problem is identified, establish a theory of probable cause by using top-to-bottom or bottom-to-top OSI layer checks to narrow the likely fault.

Question

Which of the following steps of the troubleshooting methodology would most likely include checking through each level of the OSI model after the problem has been identified?
  1. Establish a theory. Correct Answer
  2. Implement the solution.
  3. Create a plan of action.
  4. Verify functionality.

Key Takeaways

It tests the order of CompTIA's troubleshooting steps and the trap of placing OSI-layer diagnosis in verification or planning instead of theory formation.

In CompTIA's troubleshooting methodology, systematic OSI layer checks after identifying a problem belong to establishing a theory of probable cause. The correct step is A, not planning, implementation, or verification.

Common mistake: Choosing D, Verify functionality, because OSI checks sound like validation; verification happens after implementing a fix and confirms full functionality, not initial cause isolation.

Expert Analysis

Where the OSI sweep belongs The stem places us immediately after the problem has been identified. In CompTIA's troubleshooting methodology, that is the handoff from 'Identify the problem' to 'Establish a theory of probable cause.' Checking each OSI layer, either top-to-bottom from Application down or bottom-to-top from Physical up, is a structured way to generate and narrow that theory. It asks where in the communication path the failure first appears, so it is an investigative activity, not a repair or final proof. That makes A the correct step.

Why the other steps do not fit Create a plan of action (C) comes after a probable cause has been established and tested; the plan documents what will be changed, who is affected, and how to roll back. Implement the solution (B) is execution, not diagnosis. Verify functionality (D) happens after the change, when you confirm full system functionality and preventive measures. A comment arguing for D reads 'after' as proof that all diagnosis is over, but 'after the problem has been identified' marks the start of cause isolation, not the end of the repair. Another comment proposes C as a 'blueprint' for checking layers, but the methodology treats OSI layer traversal as part of forming the theory, not as the plan's change-control details.

Methodology evidence and exam signal CompTIA's published sequence is identify, establish a theory, test the theory, establish a plan, implement or escalate, and verify full functionality. The vendor guidance for establishing a theory includes considering multiple approaches such as top-to-bottom or bottom-to-top for layered technologies. Comments paraphrasing that guidance land on A; they note that OSI checks narrow possible causes before testing the theory, and only later does verification confirm the fix. If the stem said the repair was complete and the technician was checking every layer for residual issues, D could be tempting, but it does not say that. The decisive action is to use OSI layer checks to form a probable-cause theory.

Official Reference

https://www.comptia.org/blog/troubleshooting-methodology

This link opens the official vendor documentation in a new tab.

Exam Strategy

Memorize the six-step CompTIA troubleshooting sequence as an order-of-operations question. When OSI layers appear, ask whether the technician is still isolating a cause or already validating a repair; top-to-bottom or bottom-to-top checks belong to theory formation. Eliminate choices that occur after a fix is applied unless the stem explicitly says the repair is complete.

Frequently Asked Questions

Why is checking each OSI layer not part of Verify functionality?

Verification happens after a solution is implemented and confirms full system functionality. Before a fix exists, OSI-layer checks are used to isolate where communication fails and form a theory of probable cause.

What do top-to-bottom and bottom-to-top mean in OSI troubleshooting?

They are ordered approaches to the layered model: start at Application and move down, or start at Physical and move up. CompTIA lists both under establishing a theory for layered technologies such as networks.

Does Establish a theory of probable cause mean guessing the fault?

No. It means using the identified symptoms, questioning the obvious, and considering structured approaches such as OSI layer checks to narrow down the most likely cause before testing it.

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Community Votes

A
67%
D
25%
C
8%

67% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 16 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 16

  1. Nyang2 👍 13
    https://www.comptia.org/blog/troubleshooting-methodology 1. Identify the problem 2. Establish a theory of probable cause ===> Answer : A 3. Test the theory to determine the cause 4. Establish a plan of action to resolve the problem and identify potential effects 5. Implement the solution or escalate as necessary 6. Verify full system functionality and, if applicable, implement preventive measures 7. Document findings, actions, outcomes and lessons learned
  2. Dennizje 👍 6 Answered D
    Correction, it's D Verifying using the OSI model is explicitly mentioned in the troubleshooting methodology.
  3. KD603 👍 3 Answered A
    The next step after identifying the problem is establishing a theory of probable cause which can include considering multiple approaches: top-to-bottom or bottom-to-top OSI model approaches. Verify functionality would include OSI model but only after implementing the solution which hasn't happened in this question
  4. chupapi_001 👍 3 Answered A
    The correct answer is A: Establish a theory.
  5. lord_darth_vader 👍 3 Answered A
    A. Establish a theory
Show 11 more comments
  1. b82faaf 👍 2 Answered A
    A. Establish a theory According to the CompTIA troubleshooting methodology, once the problem has been identified, the next step is to establish a theory of probable cause. This involves systematically analyzing the problem and narrowing down potential causes. Checking through each level of the OSI model is a common practice at this stage to identify where the issue might be occurring.
  2. Dennizje 👍 2 Answered A
    Notice or identify problem, hypothesize possible cause, look for problem testing the possible cause, fix the problem, only then you verify the functionality.
  3. danduhman 👍 1 Answered D
    "... after the problem has been identified?" <- this portion of rhetoric suggests that only one solution is correct, as A, B, C, and D are all viable steps. With this in mind, it leaves D as the only viable step, as we ("... after the problem has been identified?") are in a "after" tense situation. "After" we have "identified" the problem, etc. Assume we did all of that, and we have fixed the issue, as we know we fixed the issue: " ... after the problem has been identified?" Considering we know we fixed the problem, which includes A, B, and C, that only leaves D, to verify.
  4. 93831b0 👍 1 Answered C
    Here is my view on this one. Designing a methodical process to check each layer of the OSI model falls under "Create a plan of action." This step involves developing a thorough plan that includes troubleshooting each OSI layer to pinpoint the issue. You can look at it like drawing up a detailed blueprint for troubleshooting. You're outlining how you'll approach each layer if necessary.
  5. 93831b0 👍 1
    Changed my mind, A. establish a theory is the step where you try to determine the most likely cause of the identified problem. You use the information you've gathered to form a hypothesis about what's going wrong. Checking through each level of the OSI model is a common technique to systematically narrow down the possible causes and pinpoint the most probable one.
  6. ec80b38 👍 1
    The reason it is said to be A is because from comptias website under establish a theory "The steps in this phase are: Questioning the obvious to identify the cause of the problem Considering multiple approaches, including top-to-bottom or bottom-to-top for layered technologies (such as networks)"
  7. Fauzto 👍 1 Answered A
    After identifying the problem, the next step in troubleshooting is to establish a theory of probable cause. This involves thinking critically and using your technical knowledge to hypothesize what might be causing the issue.
  8. Intel_Geek 👍 1 Answered D
    When verifying functionality after identifying a network problem, Checking each layer of the OSI model would be good to make sure the issue is fully resolved at every level of communication - please correct me if I am wrong
  9. Jesus_KillsBadNature 👍 1 Answered A
    A theory is built around the probable problem identified.
  10. Hayder81 👍 1
    Establish a theory chat gpt
  11. Dese_2point71 👍 1 Answered A
    A. Establish a theory.

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