troubleshooting

© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public 1Version 4.1

Troubleshooting

Working at a Small-to-Medium Business or ISP – Chapter 9

2© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

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.


Troubleshooting Methodologies and Tools Knowledge of the features, functions and devices of

each OSI or TCP/IP layer supports efficient troubleshooting


Three troubleshooting approaches:

Top-down

Bottom-up

Divide-and-conquer

Troubleshooting Methodologies and Tools


Helpful network diagrams:

Physical network topology

Logical network topology



Additional troubleshooting tools:

Documentation and baseline tools

Management system tools

Knowledge bases

Protocol analyzers



Hardware troubleshooting tools:

Cable testers

Digital multimeters

Portable network analyzers



Layer 1 problems are related to the type of technology used

Layer 1 problems can result in loss of connectivity or degraded network performance

Troubleshooting Layer 1 and Layer 2 Issues


Troubleshooting Layer 1 and Layer 2 Issues Layer 2 specifies data format and network access

Network analyzers can diagnose Layer 2 issues


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


Five common bootup errors:

POST failure

Corrupt flash image

Corrupt or missing configuration file

Memory error

Module error




Common issues causing up/down or down/down outputs:

Loose or incorrectly terminated cables

Damaged interface or cable

Improper encapsulation configuration


Use output from the show interfaces command to help identify media errors:

Excessive noise

Excessive collisions

Excessive runts

Late collisions



Troubleshooting LAN connectivity on switches:

Observe port LEDs

Verify cables

Verify configuration

Verify duplex settings



Troubleshooting WAN connectivity issues:

Use show interfaces serial output

Know the type of modem or CSU/DSU being used



Many Layer 3 problems are due to poorly designed and configured IP addressing schemes

Determine the host range in order to troubleshoot

Troubleshooting Layer 3 IP Addressing Issues


Problems resulting from poor Layer 3 planning:

Overlapping subnets

Misconfigured subnet masks

Insufficient addresses available through DHCP



DHCP issues:

Physical connectivity

Server misconfiguration

Address conflicts



NAT issues:

Incorrect designation of inside and outside interfaces

Pool misconfiguration

No route to the Internet for translated addresses



Common routing issues:

Manual route entry errors

Routing protocol configuration errors

Failures at lower OSI layers

Troubleshooting Layer 3 Routing Issues


The routing table indicates connected, dynamic, static and default routes



Issues related to RIP:

Version mismatch

Incorrect or missing network statements



Tools for troubleshooting dynamic routing issues:

TCP/IP utilities

Debug commands



Traffic filtering issues:

Firewall misconfiguration

Incorrect port assignment

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



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



Telnet provides troubleshooting access, but is an insecure protocol

SSH is a more secure method for remote device access



ICND1 exam (640-822) includes topics on networking fundamentals

Pass ICND1 to achieve CCENT certification

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



Make the commitment

Create a study plan

Practice test taking



Visit the testing center

Become familiar with the test format

Use exam tutorials and practice tests

Practice Packet Tracers and labs



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.

Summary

troubleshooting

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