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