Lecture 5.1
Troubleshooting
Problem Solving Process
Approaches to Troubleshooting
3
Trial and Error
Solve by Example
The Replacement Method
Step by Step with the OSI Model
Trial and Error
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This method requires a repeated process of:
an assessment of the problem
a guess of the solution
an implementation of the solution
a test of the results
Has a time and a place
Not always the best approach
Not to be relied on exclusively
Solve by Example
This is the process comparing something that doesn’t work with something that does and then making modifications to the non-functioning item until it performs like the functioning one.
A copy of configuration can be used as a model to base on or to modify from
Only use when the working sample has a similar environment as the machine with the problem
Don’t make changes that could cause conflicts
Be careful to not destroy data
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The Replacement Method
It is effective only if the problem source can be determined and source is a defective part
The rules of the replacement method
Narrow the list of potentially defective parts down to a few possibilities
Make sure you have the correct replacement parts on hand
Replace only on part at a time
If your first replacement does not fix the problem, reinstall the original part before replacement another part
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Step by Step with the OSI Model
Top-down
Test a problem starting from the Application layer and keep testing at each layer until the problem is resolved
Bottom-up
Test a problem starting from the physical layer and keep testing at each layer until the problem is resolved
Networks can be complex, multilayered systems, using the layered approach to troubleshoot can be helpful
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Step by Step with the OSI Model Scenario
A user at PC A complains that an error occurs when she tries to access files on the File Server, but users at PCs B and C are not having similar problems. Using the File Explorer to browse the network, no devices are shown.
A bottom-up approach:
Check the cabling, is it damaged? Is it plugged in? Are the link lights on on the NIC?
Check the NIC driver is functional and installed correctly
Check the addressing settings
Use ping and tracert
A
B
C
File server
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Some Common Problems
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Default gateway not set, or is incorrect?
The default gateway is the address of the router that the PC will use to access the outside world. It must be physically and logically connected
Subnetting error
Is the device in the same subnet as other devices on the local link?
Is the subnet mask correct?
Cabling
Not connected
Damaged
Incorrect cable used
Routing
Wrong entries
Missing entries
Summarisation error
Router interfaces
Interfaces are disabled by default
Clock rate on DCE interfaces must be set
Network Troubleshooting Tools
ICMP-based utilities
Using ping for connectivity troubleshooting
Run ipconfig /all
tracert/traceroute
Network monitors
Protocol analyser
Cable testers
The Internet
Vendor support services
Experience
Network Documentation
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Using ping for connectivity troubleshooting
Run ipconfig /all to display all related addressing information
Ping the loopback address to verify that TCP/IP stack is functioning correctly
Ping the local IP address to verify the computer’s capability to receive ICMP packets
Ping the default gateway to identify the scope of the network problem
Ping the target host
Ping DNS servers
Query the name service for information about the specified IP address or domain name, e.g. nslookup
Prototyping in Packet Tracer
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Reference
Chapter 13 Troubleshooting and Support, G. Tomsho, Guide to Networking Essentials, 7th Edition, Cengage Learning
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Define the
problem and the
scope
Gather
information
Consider possible
causes
Devise a possible
solution
Implement the
solution Test the solution
is the problem
solved?
Document the
solution
Devise
preventative
measures
No
Yes
Define the
problem and the
scope
Gather
information
Consider possible
causes
Devise a possible
solution
Implement the
solution
Test the solution
is the problem
solved?
Document the
solution
Devise
preventative
measures
No
Yes
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