How to troubleshoot IP connectivity and Cisco IOS CEF

Troubleshooting problems with Cisco Express Forwarding's network switching is easier than you might think. In this chapter from Cisco Express Forwarding, learn how to identify and fix common problems, many of which don't require extensive knowledge of Cisco's architecture and platforms.

Cisco Express Forwarding (CEF) troubleshooting can be tedious, laborious, and difficult. However, most instances of CEF troubleshooting do not require detailed Cisco IOS architecture and platform (hardware) architecture knowledge. For example, many CEF issues are found in two or three steps of troubleshooting. In addition, many issues that appear to be CEF-related end up being a result of a misconfiguration or inoperable end device.

The first section of this chapter presents the general troubleshooting used on Cisco IOS routers and switches as a first step in troubleshooting IP connectivity problems. CEF occasionally is the scapegoat for IP connectivity problems, and this chapter helps you verify whether CEF is the root cause of a particular IP connectivity problem.

This chapter does not delve into platform specifics of troubleshooting CEF. The chapter simply approaches troubleshooting from a CEF software-switching and command-line interface (CLI) perspective. Most mid- to high-end routers and all Catalyst switches support distributed CEF (dCEF), or hardware switching.

This chapter begins the CEF troubleshooting for all Cisco platforms, including the Cisco 2600, 3700, 7500, 12000, and Catalyst 6500. Chapter 5, "Understanding Packet Switching on the Cisco Catalyst 6500 Supervisor 720," goes into further detail for additional platform and hardware troubleshooting of CEF for the Cisco Catalyst 6500.

The chapter concludes with a table of the basic CEF troubleshooting commands.

Troubleshooting IP Connectivity

As mentioned in the introduction, CEF is a common scapegoat for IP connectivity issues. As such, when approaching an IP connectivity issue, keep an open mind about the root cause of the issue.

This section reviews the methodology for troubleshooting IP connectivity issues, which leads to identifying and troubleshooting CEF issues.

NOTE This chapter is based on Cisco IOS Release 12.3. All the command output presented in this chapter might not be available in previous releases of Cisco IOS. Check the command reference for your specific Cisco IOS version to verify whether a specific command is supported.

The best approach in troubleshooting is to build a troubleshooting plan. Flow charts simplify troubleshooting because they present a stepwise approach to troubleshooting. The following list briefly outlines the first steps in troubleshooting IP connectivity issues and Cisco IOS CEF:

Step 1 Accurately describe the problem.
Step 2 Scope the network topology.
Step 3 Review the Open Systems Interconnection (OSI) model.

(a) Verify the physical layer.
(b) Verify the Layer 2 layer topology.

Step 4 Verify the Address Resolution Protocol (ARP) table.
Step 5 Verify the IP routing table.
Step 6 Verify the CEF Forwarding Information Base (FIB) table.
Step 7 Verify the adjacency table.
Step 8 Conduct hardware-specific troubleshooting.

The chapter concludes with a table of the basic CEF troubleshooting commands.

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.

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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