Basic IP Connectivity and Troubleshooting in Cisco Express Forwarding > Troubleshooting IP Connectivity. This chapter covers the following topics: Troubleshooting IP connectivity. Troubleshooting punt adjacencies. Understanding CEF error messages. Troubleshooting commands reference. 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 (d. CEF), or hardware switching. This chapter begins the CEF troubleshooting for all Cisco platforms, including the Cisco 2.

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Catalyst 6. 50. 0. Chapter 5, . 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. The best approach in troubleshooting is to build a troubleshooting plan.

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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. 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. Scoping the Network Topology.

It is nearly impossible to troubleshoot any type of CEF issue or network connectivity issue without a network diagram that depicts IP addresses, IP routes, devices such as firewalls and switches, and so on. Troubleshooting IP connectivity problems without the aid of a visual topology is nearly impossible unless you can localize the issue to a specific router or switch. In large IP routing scenarios, a network topology is required to troubleshoot connectivity problems. Generally, both logical and physical topologies aid in troubleshooting. Figure 4- 1 illustrates a sample physical topology. Reviewing the OSI Model for Troubleshooting.

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. 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. Switch#show interfaces Gigabit. Ethernet 3/6. Gigabit. Ethernet. 3/6 is up, line protocol is up (connected). Hardware is Gigabit Ethernet Port, address is 0.

MTU 1. 50. 0 bytes, BW 1. Kbit, DLY 1. 0 usec.

Encapsulation ARPA, loopback not set. Keepalive set (1.

Full- duplex, 1. 00. Mb/s, link type is auto, media type is 1. Base. SX. input flow- control is on, output flow- control is off. ARP type: ARPA, ARP Timeout 0. Last input 0. 0: 0.

Last clearing of . 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: 8. D, 8. 02. 1w, and 8.

Spanning Tree Protocols. Hot Standby Routing Protocol (HSRP)Virtual Router Redundancy Protocol (VRRP)Ether. Channeling (port channeling)Compression (software- or hardware- based)Encryption (software- or hardware- based)Firewalls.

Virtual Private LAN Services (VPLS)Any Transport over Multiprotocol Label Switching (ATo. M)Consult the list for applicable features in your network topology and refer to the Cisco. Mixcraft 6 Full Version Seriale there. 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.