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Network+ Guide to Networks,
Fourth Edition
Chapter 12
Troubleshooting Network Problems
Troubleshooting Methodology
• Steps for troubleshooting network problems:
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–
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Identify symptoms and potential causes
Identify affected area
Establish what has changed
Select most probable cause
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Verify user competency
Re-create problem
Verify physical integrity of network connection
Verify logical integrity of network connection
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Troubleshooting Methodology
(continued)
• Steps for troubleshooting network problems
(continued):
– Implement an action plan and solution and be
prepared for all potential effects
– Test the result
– Identify results and effects of solution
– Document solution and process
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Establish What Has Changed
• Questions to help pinpoint problems resulting from
a network change:
– OS or configuration on a server, workstation, or
connectivity device changed?
– New components added to server, workstation, or
connectivity device?
– Old components removed from server, workstation,
or connectivity device?
– New users or segments added to the network?
– Server, workstation, or connectivity device moved
from previous location to new location?
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Select the Most Probable Cause:
Verify User Competency
• Ensure that human error is not source of problem
• Problems caused by human error usually simple to
solve
• Best way to verify that a user is performing network
tasks correctly is to watch them
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Re-create the Problem
• Try to reproduce symptoms both while logged on
as the user who reported the problem and while
logged on under a privileged account
• Questions to help determine whether a problem’s
symptoms are truly reproducible:
– Make symptoms recur every time?
– Make symptoms recur some of the time?
– Symptoms happen only under certain
circumstances?
– Symptoms consistent no matter how many and
which programs or files user has open?
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Verify Physical Connectivity
• Many network problems occur at Physical layer
– Symptoms of Physical Layer problems: continuous
or intermittent inability to connect to network and
perform network related functions
– Possible causes of Physical Layer problems:
• Segment or network lengths exceeding IEEE
maximum standards
• Noise affecting wireless or wire-bound signal
• Improper terminations, faulty connectors, loose
connectors, or poorly crimped connections
• Damaged cables
• Faulty NICs
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Implement an Action Plan and Solution
Including Potential Effects
• Scope: assess scope of solution before
implementing
• Tradeoffs: solution may restore functionality for one
group of users, but remove it for others
• Security: solution may inadvertently result in
addition or removal of network access or resource
privileges for a user or group of users
• Scalability: does solution position network for
additions and enhancements later on
• Cost: if solution requires significant software or
hardware cost, weigh options carefully
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Test the Results
• Must verify that problem solved properly
– Type of testing depends on solution
– Depends on area affected by problem
• May not be able to test solution immediately after
implementing it
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Identify the Results and Effects
of the Solution
• Should be able to determine how and why solution
was successful and effects on users and
functionality
• Want to avoid creating unintended, negative
consequences as result of solution
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Document the Solution and Process
• Always record symptoms and cause(s) of a
problem and solution
– Impossible to remember circumstances of each
incident
– Networking personnel frequently change jobs
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Help to Prevent Future Problems
• Not all problems are preventable, but many can be
avoided
– Perform regular network health checks
• Even continual network monitoring
• When planning or upgrading a network, think about
how good network designs and policies can
prevent later problems
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Troubleshooting Tools:
Crossover Cable
• Transmit and receive wire pairs in one connector
are reversed
– Directly interconnect two nodes without using an
intervening connectivity device
– Useful for quickly and easily verifying that node’s
NIC is transmitting and receiving signals properly
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Tone Generator and Tone Locator
• Telecommunications closets often disorganized
and poorly documented
• Tone generator: Issues signal on wire pair
• Tone locator: Emits tone when it detects electrical
activity on wire pair
• Cannot be used to determine cable characteristics
– Such as whether it has defects or whether its length
exceeds IEEE standards for a certain type of
network
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Tone Generator and Tone Locator
(continued)
Figure 12-4: Use of a tone generator and tone locator
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Multimeter
• Can measure many characteristics of an electric
circuit, including its resistance and voltage
• Voltmeter: measures voltage of electric current
• Ohmmeter: measures resistance
– Every type of wire has different resistance
characteristics
• Impedance: resistance that contributes to
controlling signal
– Telltale factor for ascertaining location of faults in a
cable
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Cable Continuity Testers
• Test whether cable carrying signal to destination
– When used on copper-based cables, applies small
voltage to each conductor at one end of a cable
• Checks whether voltage detectable at other end
• Two parts:
– Base unit: generates voltage
– Remote unit: detects voltage
• Some verify that wires in UTP or STP cables paired
correctly and are not shorted, exposed, or crossed
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Cable Continuity Testers (continued)
Figure 12-6: Cable continuity testers
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Cable Performance Testers
• Accomplishes same continuity and fault tests as a
continuity tester, and more
• Performance testers perform following tasks:
– Measure distance to connectivity device, termination
point, or cable fault
– Measure attenuation along cable
– Measure near-end crosstalk between wires
– Measure termination resistance and impedance
– Pass/fail ratings for CAT 3, 5, 5e, 6, or 7 standards
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Cable Performance Testers
(continued)
• Performance testers perform following tasks
(continued):
– Store and print cable testing results or directly save
data
– Graphically depict cable’s attenuation and crosstalk
characteristics over length of cable
• Time domain reflectometer (TDR): issues signal on
cable and measures way signal reflects
• Optical time domain reflectometer (OTDR): used
for fiber optic cables
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Cable Performance Testers
(continued)
Figure 12-7: A performance tester
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Network Monitors
• Software tool that continually monitors network
traffic from a server or workstation
– Typically can interpret up to Layer 3
– Determines protocols passed by each frame
– Can’t interpret data inside frames
• Many available programs:
– Microsoft Network Monitor ships with Windows
Server 2003
– Novell NETMON comes with NetWare 5.x and 6.x
– Many others available
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Protocol Analyzers
• Capture traffic and analyze frames
– Typically up to Layer 7
– Can interpret payload portion of frames
• Can generate traffic in attempt to reproduce
network problem and monitor multiple network
segments simultaneously
• Hardware-based protocol analyzers exist
– Sole job is to identify and assess network problems
– Tailored to particular type of network
• May collect more information than can be
processed
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Protocol Analyzers (continued)
Figure 12-8: Traffic displayed by protocol type
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Wireless Network Testers
• To test wireless connections, stations, or APs,
need tools that contain wireless NICs and run
wireless protocols
• Programs exist that can scan for wireless signals
over a geographical range and discover all APs
and wireless stations transmitting in area
– Determine whether AP functioning properly
– Determine whether AP positioned correctly
– Determine whether stations and APs are
communicating over proper channels within a
frequency band
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Wireless Network Testers (continued)
Figure 12-10: Wireless network testing tool
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