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Network+ Guide to Networks, Fourth Edition
Chapter 12
Troubleshooting Network Problems
Objectives
• Describe the steps involved in an effective
troubleshooting methodology
• Follow a systematic troubleshooting process to
identify and resolve networking problems
• Document symptoms, solutions, and results when
troubleshooting network problems
• Use a variety of software and hardware tools to
diagnose problems
Troubleshooting Methodology
• Steps for troubleshooting network problems:
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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
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
Identify the Symptoms and Potential Causes
• Questions that may help identify non-obvious
symptoms:
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Access to network affected?
Network performance affected?
Data and/or programs affected?
Only certain network services affected?
Problems include local application, networked
application, or multiple networked applications?
– Specific error messages reported?
– One user or multiple users affected?
– Symptoms manifested consistently?
Identify the Affected Area
Figure 12-1: Identifying the area affected by a problem
Identify the Affected Area (continued)
Figure 12-1 (cont): Identifying the area affected by a problem
Identify the Affected Area (continued)
Figure 12-2: Identifying the chronological scope of a problem
Identify the Affected Area (continued)
Figure 12-2 (cont): Identifying the chronological scope of a problem
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?
Establish What Has Changed (continued)
• Questions to help pinpoint problems resulting from a
network change (continued):
– Was a server, workstation, or connectivity device
replaced?
– Was new software installed on a server, workstation, or
connectivity device?
– Was old software removed from a server, workstation, or
connectivity device?
Establish What Has Changed (continued)
• Two ways to react to problems caused by network
changes:
– Attempt to correct problem
– Attempt to reverse change and restore hardware or
software to previous state
• Complete network change records should be kept
– Make available to staff members
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
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:
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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?
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
Verify Physical Connectivity (continued)
• Diagnosing Physical Layer problems:
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Device turned on?
NIC properly inserted?
For wireless NICs, antenna turned on?
Device’s network cable properly connected to both NIC
and wall jack?
– Patch cables properly connect punch-down blocks to
patch panels and patch panels to hubs or switches?
Verify Physical Connectivity (continued)
• Diagnosing Physical Layer problems (continued):
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Hub, router, or switch properly connected to backbone?
Cables in good condition?
Connectors in good condition and properly seated?
Network lengths conform to IEEE 802 specifications?
Devices configured properly to work with network type or
speed?
Verify Physical Connectivity (continued)
• Swapping equipment:
– If you suspect problem lies with a network component,
easy to test theory by exchanging for a functional
component
– Cables, ports, data jacks, network adapters
– Difficult to swap routers and switches
Verify Physical Connectivity (continued)
Figure 12-3: Verifying physical connectivity
Verify Physical Connectivity (continued)
Figure 12-3 (cont): Verifying physical connectivity
Verify Logical Connectivity
• Questions to help identify logical connectivity
problems:
– Error messages reference damaged or missing files or
device drivers?
– Error messages reference malfunctioning or insufficient
resources?
– OS, configuration, or application been recently changed,
introduced, or deleted?
– Problem occurs with only one application or a few, similar
applications?
– Problem happens consistently?
– Problem affects single user or one group of users?
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
Implement an Action Plan and Solution
Including Potential Effects (continued)
• Use vendor information: nothing to lose by referring
to manual, except a little time
– Manuals, online information, technical support
• Implement the solution:
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Collect all documentation about problem’s symptoms
Make backups, keep old parts, print configurations
Perform change, replacement, move, or addition
Test solution
Clean up
Document solution and results
Revisit problem later
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
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
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
Staff Involved in Troubleshooting
• Many staff members may contribute to
troubleshooting network problems
– Help desk analysts often first-level support
• Creates record for incident and attempts to diagnose problem
– Second-level support analyst has specialized knowledge
in one or more aspects of a network
– Most help desks include a help desk coordinator
• Ensures analysts divided into correct teams, schedules shifts,
maintains infrastructure to enable analysts to better perform their
jobs
Record Problems and Resolutions
• Call tracking system: software used for documenting
problems
– If no call tracking system, should keep records in
electronic form
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Name, department, phone number of problem originator
Software- or hardware-related problem?
Software package or device/component to which it pertains
Symptoms of problem
Name/telephone number of network support contact
Time spent troubleshooting
Resolution
Notify Others of Change
• Record resolution in call tracking system
• Notify others of solution and what, if anything,
needed to change to fix problem
– Alerts others about problem and solution
– Notifies others of network changes made, in case they
affect other services
• Change management system: process or program
that provides support personnel with centralized
means of documenting network changes
Notify Others of Change (continued)
• Types of changes that network personnel should
record in change management system:
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Adding or upgrading software or hardware
Changing network properties of network device
Increasing or decreasing rights for group of users
Physically moving networked devices
Moving user accounts, files, and directories
Making changes in processes
Making changes in vendor policies or relationships
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
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
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
Tone Generator and Tone Locator (continued)
Figure 12-4: Use of a tone generator and tone locator
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
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
Cable Continuity Testers (continued)
Figure 12-6: Cable continuity testers
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
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
Cable Performance Testers (continued)
Figure 12-7: A performance tester
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
Network Monitors (continued)
• Network monitoring tools perform following
functions:
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Continuously monitor network traffic on segment
Capture network data transmitted on segment
Capture frames sent to or from specific node
Reproduce network conditions by transmitting selected
amount and type of data
– Generate statistics about network activity
Network Monitors (continued)
• Some network monitoring tools can also:
– Discover all network nodes on a segment
– Establish a baseline
• Record of how network operates under normal conditions,
including performance, collision rate, utilization rate, and so on
– Store traffic data and generate reports
– Trigger alarms when traffic conditions meet preconfigured
conditions
Network Monitors (continued)
• Data errors that can be distinguished:
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Local collisions
Late collisions
Runts
Giants
Jabber
Negative frame sequence checks
Ghosts
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
Protocol Analyzers (continued)
Figure 12-8: Traffic displayed by protocol type
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
Wireless Network Testers (continued)
• Some programs can capture data transmitted
between stations and APs
– Useful for troubleshooting wireless connection problems
• Some programs contain a spectrum analyzer
– Assess quality of wireless signal
– Ascertain where noise (or interference) is greatest
Wireless Network Testers (continued)
• Capabilities common to wireless network testers:
– Identify transmitting APs and stations and channels over
which they are communicating
– Measure signal strength from and determine range of AP
– Indicate effects of attenuation, signal loss, and noise
– Interpret signal strength information
Wireless Network Testers (continued)
• Capabilities common to wireless network testers
(continued):
– Ensure proper association and reassociation when moving
between APs
– Capture/interpret traffic exchanged between wireless APs
and stations
– Measure throughput and assess data transmission errors
– Analyze the characteristics of each channel within a
frequency band to indicate the clearest channels
Wireless Network Testers (continued)
Figure 12-10: Wireless network testing tool
Summary
• Key to solving network problems is to approach
them methodically and logically, using your
experience to inform your decisions, and knowing
when to ask for someone else’s help
• First step in troubleshooting is identifying the
symptoms and potential causes for a problem
• Second step in troubleshooting is to identify the
affected area
• At each point in the troubleshooting process, stop to
consider what kind of changes have occurred on the
network that might have created a problem
Summary (continued)
• Based on an analysis of the symptoms and how
changes might have affected the network, select a
probable cause for the problem
• After you have identified the probable cause,
implement an action plan and your solution, while
considering the potential effects of the solution
• After implementing your solution, test your result to
ensure that you solved the problem and haven’t
created new problems
• Next identify the effects and results of your solution
Summary (continued)
• Last step in troubleshooting is to document the
solution and the process of solving the problem
• A tone generator and tone locator are used to
identify the terminating location of a wire pair
• A multimeter is a simple device that can measure the
voltage, resistance, impedance, and other
characteristics of an electrical circuit
• Basic cable continuity testers determine whether
your cabling can provide connectivity
Summary (continued)
• A network monitor is a software-based tool that
monitors network traffic from a server or
workstation attached to the network
• Protocol analyzers can typically interpret data up to
Layer 7 of the OSI Model
• Wireless network testing tools can be dedicated
instruments or software that run on a workstation
(usually a laptop)