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

3 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Troubleshooting Methodologies and Tools  Knowledge of the features, functions and devices of each OSI or TCP/IP layer supports efficient troubleshooting

4 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Three troubleshooting approaches:  Top-down  Bottom-up  Divide-and-conquer Troubleshooting Methodologies and Tools

5 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Helpful network diagrams:  Physical network topology  Logical network topology Troubleshooting Methodologies and Tools

6 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Additional troubleshooting tools:  Documentation and baseline tools  Management system tools  Knowledge bases  Protocol analyzers Troubleshooting Methodologies and Tools

7 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Hardware troubleshooting tools:  Cable testers  Digital multimeters  Portable network analyzers Troubleshooting Methodologies and Tools

8 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public  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

9 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Troubleshooting Layer 1 and Layer 2 Issues  Layer 2 specifies data format and network access  Network analyzers can diagnose Layer 2 issues

10 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public 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

11 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Five common bootup errors:  POST failure  Corrupt flash image  Corrupt or missing configuration file  Memory error  Module error Troubleshooting Layer 1 and Layer 2 Issues

12 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public 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

13 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Use output from the show interfaces command to help identify media errors:  Excessive noise  Excessive collisions  Excessive runts  Late collisions Troubleshooting Layer 1 and Layer 2 Issues

14 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Troubleshooting LAN connectivity on switches:  Observe port LEDs  Verify cables  Verify configuration  Verify duplex settings Troubleshooting Layer 1 and Layer 2 Issues

15 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Troubleshooting WAN connectivity issues:  Use show interfaces serial output  Know the type of modem or CSU/DSU being used Troubleshooting Layer 1 and Layer 2 Issues

16 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public  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

17 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Problems resulting from poor Layer 3 planning:  Overlapping subnets  Misconfigured subnet masks  Insufficient addresses available through DHCP Troubleshooting Layer 3 IP Addressing Issues

18 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public DHCP issues:  Physical connectivity  Server misconfiguration  Address conflicts Troubleshooting Layer 3 IP Addressing Issues

19 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public NAT issues:  Incorrect designation of inside and outside interfaces  Pool misconfiguration  No route to the Internet for translated addresses Troubleshooting Layer 3 IP Addressing Issues

20 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Common routing issues:  Manual route entry errors  Routing protocol configuration errors  Failures at lower OSI layers Troubleshooting Layer 3 Routing Issues

21 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public  The routing table indicates connected, dynamic, static and default routes Troubleshooting Layer 3 Routing Issues

22 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Issues related to RIP:  Version mismatch  Incorrect or missing network statements Troubleshooting Layer 3 Routing Issues

23 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Tools for troubleshooting dynamic routing issues:  TCP/IP utilities  Debug commands Troubleshooting Layer 3 IP Addressing Issues

24 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Traffic filtering issues:  Firewall misconfiguration  Incorrect port assignment Troubleshooting Layer 4 and Upper Layer Issues

25 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public  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 Troubleshooting Layer 4 and Upper Layer Issues

26 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public  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 Troubleshooting Layer 4 and Upper Layer Issues

27 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public  Telnet provides troubleshooting access, but is an insecure protocol  SSH is a more secure method for remote device access Troubleshooting Layer 4 and Upper Layer Issues

28 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public  ICND1 exam ( ) includes topics on networking fundamentals  Pass ICND1 to achieve CCENT certification Preparing for Cisco Certification

29 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public What certification exams measure:  Knowledge: facts recalled from memory  Skills: interacting with Cisco networking devices  Abilities: scenario-based or simulation tasks Preparing for Cisco Certification

30 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public  Make the commitment  Create a study plan  Practice test taking Preparing for Cisco Certification

31 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public  Visit the testing center  Become familiar with the test format  Use exam tutorials and practice tests  Practice Packet Tracers and labs Preparing for Cisco Certification

32 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public  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

33 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public