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Cisco CCNA Cert Guide: Troubleshooting IPv4 Routing Part II

Contents

  1. Foundation Topics
  2. Review Activities

Chapter Description

Wendell Odom helps aspiring CCNAs to troubleshoot network problems that have been narrowed down from larger parts of the network. He shows how to look for the root causes: the causes of network problems that have specific solutions that, once a change is made, will solve the original problem.

Review Activities

Chapter Summary

  • Many problems can occur on the host or the default router, or between the two.
  • A typical IPv4 host gets its four key IPv4 settings either statically or dynamically.
  • To begin troubleshooting the host, use the ipconfig or ifconfig command to ensure IPv4 settings correctly match. If you are using DHCP, the ipconfig or ifconfig command will enable you to see whether DHCP has failed to learn any of the IPv4 settings.
  • The host and router should attach to the exact same subnet with the same subnet ID and same range of IP addresses.
  • When a host lists the wrong IP addresses for the DNS servers, the user actions that require name resolution will fail.
  • Any network testing with commands like ping and traceroute fails when using names but works when using IP addresses when there is a problem with the DNS settings.
  • Having a host that lists the wrong IP address as its default router will cause network problems.
  • The default router’s configuration can cause problems in a network that may stem from mismatched VLAN trunking configurations, DHCP relay issues, router LAN interface, and LAN issues.
  • If a router omits the ip helper-address command on a LAN interface, DHCP fails for those clients.
  • Another problem with network connectivity can exist with the routing or how a router forwards a packet.
  • The show ip route command plays a huge role in troubleshooting IP routing and IP routing protocol problems.
  • In some cases, overlapping routes cause a problem, while in other cases, the overlapping routes are just a normal result of using some feature.

Review All the Key Topics

Review the most important topics from this chapter, noted with the Key Topic icon. Table 5-4 lists these key topics and where each is discussed.

keytopic.jpg

Table 5-4. Key Topics for Chapter 5

Key Topic Element

Description

Page Number

List

Two root causes of DNS problems.

136

List

The rules for configuring ROAS.

138

List

Items to verify for switch trunking configuration to match a router’s ROAS configuration.

140

List

Conditions that must be true for DHCP messages to be able to flow from a client to a DHCP server.

141

Table 5-1

Common reasons why router LAN interfaces are not up/up.

142

Definition

When more than one route matches a packet’s destination address, the router uses the “best” (most specific) route.

143

List

Types of overlapping IP address configuration issues that IOS can and cannot recognize.

149

Complete the Tables and Lists from Memory

Print a copy of DVD Appendix D, “Memory Tables,” or at least the section for this chapter, and complete the tables and lists from memory. DVD Appendix E, “Memory Tables Answer Key,” includes completed tables and lists to check your work.

Definitions of Key Terms

After your first reading of the chapter, try to define these key terms, but do not be concerned about getting them all correct at that time. Chapter 22 directs you in how to use these terms for late-stage preparation for the exam.