Troubleshooting Methodologies

NET-150

What is Troubleshooting

In general terms:

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What do you think are the stages of troubleshooting?

Troubleshooting Process

Diagnosis is the process of identifying the nature and cause of a problem.

• Gathering information
• Analyzing information
• Eliminating possible causes
• Formulating a hypothesis
• Testing the hypothesis
• Solving the problem

Troubleshooting Example

The light in your kitchen is not working.

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What would you do?

1. Gather info:
• Are other lights working?
• Are other electric devices in the kitchen working?
• Observe anything unusual?
2. Analyze: What conclusions can you draw from the info

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• Eliminate: Can you eliminate some possible causes?
• For example, if other lights work, you know that the house has power

Troubleshooting Example cont…

Troubleshooting kitchen light…

4. Hypothesis: Based on analysis – what is a likely cause?
• Did the bulb burn out? Is the light fixture broken? Bad switch?, Blown circuit in kitchen?
5. Testing Hypothesis: Test the most likely (or easiest to test) first
• Swap out the bulb
• Try bulb in other fixture
• Use tools to test switch/fixture
6. Solving the Problem – the light works!

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6 Troubleshooting Approaches

It all about strategy, organization, and planning!

1. The top-down approach

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• The bottom-up approach

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• The divide-and-conquer approach

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• The follow-the-path approach

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• The spot-the-differences approach

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• The move-the-problem approach

Top Down Approach

The goal of the top-down approach is to find the highest OSI layer that is still working.

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What if Browser can’t

load the webpage?

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1. Top Down Approach

What if Browser can’t

load the webpage?

• Can it connect to other pages on that server? (Layer 7)
• Can you netcat to port 80 (Layer 4)
• Can you ping the server (Layer 3)
• Can you ping your router (layer 2/3)..
• Is your cable ok? (Layer 1)

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2. Bottom - Down Approach

What if Browser can’t

load the webpage?

Switch it up!

• Is your cable ok? (Layer 1)
• Can you ping your router (layer 2/3)…
• Can you ping the server (Layer 3)
• Can you netcat to port 80 (Layer 4)
• Can it connect to other pages on that server? (Layer 7)

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3. Divide and Conquer Approach

On larger or more complex systems, it can be helpful to try and isolate generally where the problem is – and then use another approach

• Can the Web Server ping the router?
• Yes, problem likely client side

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• Can the client ping router?
• Yes, the problem is server-side

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4. Follow the Path

A variation on Divide and Conquer

• Useful when trying to find a problem across multiple networks and devices
• Start on one side and test each hop/connection along the way
• For example, a command like traceroute/tracert will identify issues along the path

5. Spot the Differences

Spotting-the-differences approach by:

• Comparing configurations, software versions, hardware, or other device properties between working and nonworking situations
• Try to resolve the problem by changing the nonoperational elements to be consistent with the working ones

5. Spot the Differences

Spotting-the-differences approach:

• For Example:
• show running-config in Cisco IOS

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6. Move the Problem Approach

Physically swap components and observe:

• Does the problem stay in place?
• Does the problem move with the component, or disappear entirely?

Can use to isolate the problem and scope it to a single element

• Then, focus further troubleshooting on that element

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For example:

• Does this light bulb work in a different fixture?

6. Move the Problem Approach

What do you think are some examples of “Move the Problem” in Networking

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• Try connecting to a different physical port on a switch or wall jack
• Wireless connection to a different Access Point or SSID
• Switch out a network cable
• Try connecting to a remote server through a different ISP

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Takeaways

No one approach will work in every situation, but…

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Stay organized and use a method (or combination of methods)!!!

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When a problem arises – take a minute and think about troubleshooting.

Develop a plan, even if it is simple and quick!

Takeaways…

In future classes…

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If you state:

• “It Doesn’t Work”

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Faculty Response:

• “What have you done to troubleshoot it”

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Learning to have an organized approach to troubleshooting is key!