Troubleshooting
Working at a Small-to-Medium Business or ISP –
Chapter 9
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Objectives
 Use the OSI model as a framework for troubleshooting
network problems.
 Identify and correct problems with hardware and operation at
Layer 1 and Layer 2.
 Troubleshoot IP addressing problems, including subnet
mask, host range errors, DHCP and NAT issues.
 Identify and correct problems with RIPv2 configuration and
implementation.
 Explain possible causes of problems occurring with user
applications and how to recognize symptoms of DNS
failures.
 Create a plan to prepare to take the ICND1 examination in
order to obtain a CCENT certification.
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Troubleshooting Methodologies and Tools
 Knowledge of the features, functions and devices of
each OSI or TCP/IP layer supports efficient
troubleshooting
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Troubleshooting Methodologies and Tools
Three troubleshooting approaches:
 Top-down
 Bottom-up
 Divide-and-conquer
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Troubleshooting Methodologies and Tools
Helpful network diagrams:
 Physical network topology
 Logical network topology
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Troubleshooting Methodologies and Tools
Additional troubleshooting tools:
 Documentation and baseline tools
 Management system tools
 Knowledge bases
 Protocol analyzers
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Troubleshooting Methodologies and Tools
Hardware troubleshooting tools:
 Cable testers
 Digital multimeters
 Portable network analyzers
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Troubleshooting Layer 1 and Layer 2 Issues
 Layer 1 problems are related to the type of technology
used
 Layer 1 problems can result in loss of connectivity or
degraded network performance
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Troubleshooting Layer 1 and Layer 2 Issues
 Layer 2 specifies data format and network access
 Network analyzers can diagnose Layer 2 issues
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Troubleshooting Layer 1 and Layer 2 Issues
 Observe console messages during the boot sequence
 Use Cisco IOS CLI show commands to verify
operational status
 When boot problems cause a network outage, use
substitution to restore service
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Troubleshooting Layer 1 and Layer 2 Issues
Five common bootup errors:
 POST failure
 Corrupt flash image
 Corrupt or missing configuration file
 Memory error
 Module error
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Troubleshooting Layer 1 and Layer 2 Issues
Common issues causing up/down or down/down outputs:
 Loose or incorrectly terminated cables
 Damaged interface or cable
 Improper encapsulation configuration
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Troubleshooting Layer 1 and Layer 2 Issues
Use output from the show interfaces command to help
identify media errors:
 Excessive noise
 Excessive collisions
 Excessive runts
 Late collisions
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Troubleshooting Layer 1 and Layer 2 Issues
Troubleshooting LAN connectivity on switches:
 Observe port LEDs
 Verify cables
 Verify configuration
 Verify duplex settings
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Troubleshooting Layer 1 and Layer 2 Issues
Troubleshooting WAN connectivity issues:
 Use show interfaces serial output
 Know the type of modem or CSU/DSU being used
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Troubleshooting Layer 3 IP Addressing Issues
 Many Layer 3 problems are due to poorly designed and
configured IP addressing schemes
 Determine the host range in order to troubleshoot
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Troubleshooting Layer 3 IP Addressing Issues
Problems resulting from poor Layer 3 planning:
 Overlapping subnets
 Misconfigured subnet masks
 Insufficient addresses available through DHCP
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Troubleshooting Layer 3 IP Addressing Issues
DHCP issues:
 Physical connectivity
 Server misconfiguration
 Address conflicts
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Troubleshooting Layer 3 IP Addressing Issues
NAT issues:
 Incorrect designation of inside and outside interfaces
 Pool misconfiguration
 No route to the Internet for translated addresses
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Troubleshooting Layer 3 Routing Issues
Common routing issues:
 Manual route entry errors
 Routing protocol configuration errors
 Failures at lower OSI layers
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Troubleshooting Layer 3 Routing Issues
 The routing table indicates connected, dynamic, static
and default routes
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Troubleshooting Layer 3 Routing Issues
Issues related to RIP:
 Version mismatch
 Incorrect or missing network statements
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Troubleshooting Layer 3 IP Addressing Issues
Tools for troubleshooting dynamic routing issues:
 TCP/IP utilities
 Debug commands
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Troubleshooting Layer 4 and Upper Layer Issues
Traffic filtering issues:
 Firewall misconfiguration
 Incorrect port assignment
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Troubleshooting Layer 4 and Upper Layer Issues
 Basic connectivity must be
eliminated as the source of the
problem
 Check with the ISP to ensure
that the problem does not lie
with the remote network
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Troubleshooting Layer 4 and Upper Layer Issues
 Upper layer issues can exist
even with full network
connectivity
 Check application configuration
with specific problems
 Encryption or compression
mismatch may be an issue
 Make sure browser plug-ins have
been updated
 If more than one application is
affected, a DNS server issue
may be the problem
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Troubleshooting Layer 4 and Upper Layer Issues
 Telnet provides troubleshooting access, but is an
insecure protocol
 SSH is a more secure method for remote device
access
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Preparing for Cisco Certification
 ICND1 exam (640-822) includes topics on networking
fundamentals
 Pass ICND1 to achieve CCENT certification
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Preparing for Cisco Certification
What certification exams measure:
 Knowledge: facts recalled from memory
 Skills: interacting with Cisco networking devices
 Abilities: scenario-based or simulation tasks
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Preparing for Cisco Certification
 Make the commitment
 Create a study plan
 Practice test taking
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Preparing for Cisco Certification
 Visit the testing center
 Become familiar with the test format
 Use exam tutorials and practice tests
 Practice Packet Tracers and labs
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Summary
 Networking models provide a framework for efficient
troubleshooting.
 Good troubleshooting involves using a methodology,
such as top-down, bottom-up, or divide-and-conquer.
 The majority of Layer 3 problems result from poor or
incorrect addressing schemes.
 Firewall misconfiguration can result in problems with
upper layer protocols.
 Success on certification tests depends on building
knowledge, skills, and abilities.
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