Accurately describe the problem

Learn how to identify and describe IP connectivity problems with Cisco Express Forwarding. This tutorial is based on Cisco IOS Release 12.3.

Accurately articulating your IP connectivity problem is paramount to troubleshooting effectively. An ad hoc approach to troubleshooting is usually ineffective in resolving problems. For example, you do not go to your dentist and tell him you are in pain without describing the symptoms, such as which tooth, how often, how intense, how widespread, what causes the pain, and so on. The same premise exists with IP connectivity troubleshooting. To help yourself, you need to know as much about the issue as possible. The following questions aid you in accurately articulating your IP connectivity problem:

  • Is your IP connectivity problem isolated to a single end device or multiple end devices?
  • Is your IP connectivity problem isolated to a single router or Ethernet switch?
  • Does your IP connectivity problem exist only on end devices, or does it affect the management CLI of routers and switches as well?
  • How widespread is the problem?
  • Is the problem widespread or localized to specific area of your network topology?
  • Is the problem intermittent or consistent? For example, using the Internet Control Message Protocol (ICMP) ping utility in Cisco IOS and on end devices, are you getting intermittent responses to ICMP echo requests such as every other response, no responses, or inconsistent responses (one out of ten)?
  • Does this issue depend on packet size? If you send ICMP echo requests at different sizes, do you consistently get all your responses or does the problem vary with packet size?
  • When did the problem first occur? Were there any changes to the network at the same time the problem started occurring?

These questions aid you in articulating your IP connectivity issues. The next section describes an important next step, building the network topology.

Learn how to troubleshoot Cisco's Express Forwarding network switching technology in "Basic IP Connectivity and CEF Troubleshooting," Chapter 4 from the book Cisco Express Forwarding by Nakia Stringfield, Russ White and Stacia McKee.

Basic IP Connectivity and CEF Troubleshooting

  Accurately describe the problem
  Scope the network topology
  Review the OSI model for troubleshooting
  Verify the ARP table
  Verify the IP routing table
  Verify the CEF FIB table
  Verify the adjacency table
  Conduct hardware-specific troubleshooting

Reproduced from the book Cisco Express Forwarding. Copyright 2007, Cisco Systems, Inc. Reproduced by permission of Pearson Education, Inc., 800 East 96th Street, Indianapolis, IN 46240. Written permission from Pearson Education, Inc. is required for all other uses.

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