Troubleshooting

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

2. 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. Troubleshooting Methodologies and Tools • Knowledge of the features, functions and devices of each OSI or TCP/IP layer supports efficient troubleshooting

4. Troubleshooting Methodologies and Tools Three troubleshooting approaches: • Top-down • Bottom-up • Divide-and-conquer

5. Troubleshooting Methodologies and Tools Helpful network diagrams: • Physical network topology • Logical network topology

6. Troubleshooting Methodologies and Tools Additional troubleshooting tools: • Documentation and baseline tools • Management system tools • Knowledge bases • Protocol analyzers

7. Troubleshooting Methodologies and Tools Hardware troubleshooting tools: • Cable testers • Digital multimeters • Portable network analyzers

8. 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

9. Troubleshooting Layer 1 and Layer 2 Issues • Layer 2 specifies data format and network access • Network analyzers can diagnose Layer 2 issues

10. 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. 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

12. 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. 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

14. Troubleshooting Layer 1 and Layer 2 Issues Troubleshooting LAN connectivity on switches: • Observe port LEDs • Verify cables • Verify configuration • Verify duplex settings

15. 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

16. 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

17. Troubleshooting Layer 3 IP Addressing Issues Problems resulting from poor Layer 3 planning: • Overlapping subnets • Misconfigured subnet masks • Insufficient addresses available through DHCP

18. Troubleshooting Layer 3 IP Addressing Issues DHCP issues: • Physical connectivity • Server misconfiguration • Address conflicts

19. 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

20. Troubleshooting Layer 3 Routing Issues Common routing issues: • Manual route entry errors • Routing protocol configuration errors • Failures at lower OSI layers

21. Troubleshooting Layer 3 Routing Issues • The routing table indicates connected, dynamic, static and default routes

22. Troubleshooting Layer 3 Routing Issues Issues related to RIP: • Version mismatch • Incorrect or missing network statements

23. Troubleshooting Layer 3 IP Addressing Issues Tools for troubleshooting dynamic routing issues: • TCP/IP utilities • Debug commands

24. Troubleshooting Layer 4 and Upper Layer Issues Traffic filtering issues: • Firewall misconfiguration • Incorrect port assignment

25. 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

26. 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

27. 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

28. Preparing for Cisco Certification • ICND1 exam (640-822) includes topics on networking fundamentals • Pass ICND1 to achieve CCENT certification

29. 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

30. Preparing for Cisco Certification • Make the commitment • Create a study plan • Practice test taking

31. 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

32. 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.