300-135 TSHOOT Quick Study Notes: Troubleshooting Methods

I took the 300-135 TSHOOT exam at Cisco Live, in the hopes of renewing my Cisco certifications, but I failed. So, it's back to the books and practice exams... Here are my study notes I've been posting on Twitter (@dotonebit).

Popular troubleshooting methods:
  • top-down
  • bottom-up
  • divide and conquer
  • follow the path
  • compare configurations
  • swap components

The top-down method:
  • check the application first and move down the OSI model from there
  • if a layer is functioning, assume all lower layers are also functioning
  • example: if ping works (ICMP/Layer 4), you can assume Layers 1-3 also work

The bottom-up method:
  • investigate OSI layers starting at Layer 1
  • if all elements of a layer are good, move up to the next layer until the problem is found
  • effective approach when the problem is suspected to be physical; otherwise exhaustive

The divide-and-conquer method:
  • an "educated guess" approach
  • choose a layer based on symptoms you have gathered
  • if the layer is working, lower layers are operational and bottom-up method can begin from that point
  • if not working, use top-down method

Follow the path:
  • investigate the path the packet would take and deterministically pinpoint the failure
  • example: first check PC-SW, the check SW-RTR, then check RTR-ISP, and so on... until you find the problem

Compare configurations:
  • compare a device's configuration to the configuration of another device that is working properly
  • example: do show run and check if the devices have corresponding configurations

Component swapping:
  • if a problem's symptoms disappear after swapping out a particular component, you can conclude that the old component was faulty
  • might fix the problem but might not provide great insight into the specific issue


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