Network+ Guide to Networks, Fourth Edition Chapter 12 Troubleshooting Network Problems

Network+ Guide to Networks, Fourth Edition

Chapter 12

Troubleshooting Network Problems

Network+ Guide to Networks, 4E 2

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:– 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


Identify the Symptoms and Potential Causes

• Questions that may help identify non-obvious symptoms:– 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 (continued): Identifying the area affected by a problem



Figure 12-2: Identifying the chronological scope of a problem



Figure 12-2 (continued): 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:– 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:– 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?



• Diagnosing Physical Layer problems (continued):– 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?



• 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



Figure 12-3: Verifying physical connectivity



Figure 12-3 (continued): 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: – 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• 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:– 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:– 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



• 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



• Data errors that can be distinguished:– 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



• 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



• 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



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)

Documents

Network+ Guide to Networks, Fourth Edition Chapter 12 Troubleshooting Network Problems