Review the Open Systems Interconnection model for troubleshooting

Learn how to review the OSI model as part of next step in troubleshooting any IP connectivity issue to ensure it is a Layer 3 (network) issue.

The next step in troubleshooting any IP connectivity issue is to review the OSI model and verify that your issue is indeed a Layer 3 (network) issue. Figure 4-2 briefly reviews the OSI model. As a reader of this high-level technology book on CEF, we assume you have an understanding of the OSI model.

Figure 4-2 OSI Model

OSI model for network switching troubleshooting

The following sections start from the bottom of the OSI model and review troubleshooting physical connectivity and Layer 2 issues that can affect IP connectivity and give the appearance of a CEF issue.

Troubleshooting Physical Connectivity

An IP connectivity issue might simply be a Layer 1 (physical layer) problem. For example, if you are unable to ping a network device through a router, do not assume that you are having a CEF issue. First, ensure that the host is connected and verify that the physical layer between the host and destination is not sustaining errors. Example 4-1 illustrates sample output from a show interfaces command in Cisco IOS.

Example 4-1 Verifying the Physical Layer

In regard to the show interfaces command and verifying physical connectivity, verify that your ingress and egress interfaces are not sustaining errors such as input errors, cyclic redundancy check (CRC) errors, output errors, excessive collisions, overruns, late collisions, or output buffer failures. These types of errors can lead to intermittent or total loss of IP connectivity. Generally, physical layer issues cause intermittent connectivity if the connection has link. Layer 1 errors can be a result of a bad cable, bad port, faulty hardware, and so on.

Before proceeding with IP connectivity troubleshooting and ultimately CEF troubleshooting, you must rule out Layer 1 issues.

Troubleshooting Layer 2 Issues

The next step in troubleshooting intermittent connectivity, after physical layer (Layer 1) issues have been ruled out, is to verify that the Layer 2 topology is operating correctly. Verifying the Layer 2 topology includes, but is not limited to, verifying the spanning-tree topology, MAC address table, and Hot Standby Routing Protocol (HSRP) status in switched networks. In a point-to-point router configuration, these features are seldom used.

Nevertheless, if your topology involves aggregating and connecting routers to Ethernet switches, you must check the Layer 2 topology before assuming that CEF is the root cause of an IP connectivity issue. Because CEF is found on all Layer 3 Cisco Catalyst switches, troubleshooting Layer 2 issues is necessary when troubleshooting IP connectivity issues on these platforms.

The following list articulates the most common Layer 2 features that can affect IP connectivity when not operating in a correct state:

  • 802.1D, 802.1w, and 802.1s Spanning Tree Protocols
  • Hot Standby Routing Protocol (HSRP)
  • Virtual Router Redundancy Protocol (VRRP)
  • EtherChanneling (port channeling)
  • Compression (software- or hardware-based)
  • Encryption (software- or hardware-based)
  • Firewalls
  • Virtual Private LAN Services (VPLS)
  • Any Transport over Multiprotocol Label Switching (AToM)

Consult the list for applicable features in your network topology and refer to the Cisco.com website for more details on troubleshooting these features. Troubleshooting these features is outside the scope of this book.

Because many Cisco IOS router experts are not also Cisco IOS switch experts, router experts can find switch troubleshooting daunting. Nevertheless, you should rule out Layer 2 issues as a cause for IP connectivity and CEF issues. The most efficient way to troubleshooting Layer 2 issues is to remove redundant paths, disable as many features as possible, and isolate connectivity issues to a single host.

After the mundane Layer 1 and Layer 2 issues have been ruled out, you can finally transition to troubleshooting IP and CEF, the main focus of this book. As with any troubleshooting, you use a stepwise approach.

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

  Introduction
  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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