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CCNA 1.8: Apply troubleshooting methodologies to resolve problems (Fault isolation, resolve/escalate/verify/monitor)


Understand troubleshooting methodologies and approaches to efficiently resolve network issues.


Study Notes:

  • Troubleshooting methodology
    • Define the problem
    • Gather information
    • Analyze information
    • Eliminate potential causes
    • Propose hypothesis
    • Test hypothesis
    • Solve problem and document solution
  • Approach:
    • Top-down method: - start at the top of the OSI model
    • Bottom-up method: start at the bottom of the OSI model
    • Divide and conquer method: start in the middle of the OSI model
  • Methods:
    • Compare configuration to another similar device
    • Follow the path, checking each device in the path as you go
    • Swap components


My two cents:

I have come across many people in the industry (certified and not) who have very haphazard troubleshooting methods.  They get flustered quickly because they lose focus.  They lose focus because they assume they know what the problem is.  Then they find another clue that points them in a different direction and they go investigate that.  And then another clue and another direction.  They end up jumping around the network without ever actually ruling anything out completely.  My advice is to never assume you know what the root cause of the problem is until you've methodically isolated the issue.

I like the divide and conquer method (not a real method, but a nice mnemonic).  I have found it to be the most efficient and effective. What I mean is that once you define the problem you are trying to solve the next step is to attempt to isolate it.  Luckily for us, everything on a network is connected, which makes most troubleshooting efforts binary.  It either works or it doesn't.  Bandwidth and latency issues tend to be non-binary and more difficult to troubleshoot.  Anyways, if you are able to divide your network into segments and test the problem over a part of the network instead of the entire network than you will be able to much more efficiently isolate the problem.

For example, your coworker's device can’t reach the internet so they call you to troubleshoot it.  You know that between their device and the internet exists a switch, a router and a firewall.  You divide the network in half and test if their device can reach the router. Then you run a separate test to see if the router can get to the internet.  One of the two tests will most likely fail and the other will pass. Focus on the segment of the network that failed. Say it was the client to the router. Now divide that segment in half, which would be at the switch and test again.  Test connectivity from the client to the switch. Test the connectivity from the switch to the router. Say the client is not getting to the switch.

At this point you need to select either the client device or the switch and start working your way up the OSI model.  Is the client machine physically connected to the network with it's link light green (layer 1)? Is it displaying ARPs (layer 2)? Does it have an IP address (layer 3)?, etc. If you fail to find the problem on the client, go to the switch and check that there is a physical cable connected to the port on the switch (layer 1). Check that the status of the port is up/up, check the vlan and look for ARPs from the client (layer 2).  Put an SVI on the switch and see if you can ping the client device (layer 3). Long story short, divide and conquer to accomplish efficient fault isolation and then step up the OSI model for resolution. Don’t forget to test access to the internet (layer 7) once you think you've resolved the issue. There are times when there are multiple unrelated problems occurring simultaneously. What happens when you fix a problem and tell the customer they are good without testing it yourself?  They try to get to the internet and it looks like you didn’t fix anything. Don’t do that to yourself. Trust but verify!

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