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Network+ Guide to Networks, Fourth Edition

Chapter 12

Troubleshooting Network Problems

Network+ Guide to Networks, 4E 2

Troubleshooting Methodology

• Steps for troubleshooting network problems:– Identify symptoms and potential causes– Identify affected area– Establish what has changed– Select most probable cause

• 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


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?


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:– 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


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


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


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


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)

Figure 12-10: Wireless network testing tool

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