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Chapter 9: Troubleshooting the Network

Connecting Networks

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Chapter 99.0 Introduction

9.1 Troubleshooting with a Systematic Approach

9.2 Network Troubleshooting

9.3 Summary

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9.1 Troubleshooting with a Systematic Approach

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Chapter 9: Objectives Explain how network documentation is developed and used to

troubleshoot network issues. Describe the general troubleshooting process. Compare troubleshooting methods that use a systematic, layered

approach. Describe troubleshooting tools used to gather and analyze

symptoms of network problems. Determine the symptoms and causes of network problems using a

layered model. Troubleshoot a network using the layered model.

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Network DocumentationDocumenting the NetworkNetwork documentation is a complete set of accurate and current network documentation. This documentation includes: Configuration files, including network configuration files and end-

system configuration files Physical and logical topology diagrams A baseline performance level

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Network DocumentationNetwork Topology Diagrams

Physical TopologyDevice type

Model and manufacturer

Operating system versionCable type and

identifierCable specification

Connector typeCabling endpoints

Logical TopologyDevice identifiers

IP address and prefix lengths

Interface identifiersConnection type

DLCI for virtual circuitsSite-to-site VPNs

Routing protocolsStatic routes

Data-link protocolsWAN technologies

used

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Network DocumentationEstablishing a Network Baseline

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Network DocumentationEstablishing a Network Baseline (cont.)

Step 1. Determine what types of data to collect.

Step 2. Identify devices and ports of interest.

Step 3. Determine the baseline duration.

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Network DocumentationMeasuring Data

Commands that are useful to the network documentation process include: ping telnet show ip interface brief show ipv6 interface brief show ip route show ipv6 route show cdp neighbor detail

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Troubleshooting ProcessGeneral Troubleshooting Procedures

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Troubleshooting ProcessGathering Symptoms

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Troubleshooting ProcessQuestioning End Users

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Isolating the Issue Using Layered ModelsUsing Layered Models for Troubleshooting

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Isolating the Issue Using Layered ModelsTroubleshooting MethodsUsing the layered models, there are three primary methods for troubleshooting networks: Bottom-up Top-down Divide-and-conquer

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Isolating the Issue Using Layered ModelsTroubleshooting Methods (cont.)In addition to the systematic, layered approach to troubleshooting, there are also, less-structured troubleshooting approaches: One troubleshooting approach is based on an educated guess by

the network administrator, based on the symptoms of the problem. Another approach involves comparing a working and nonworking

situation, and spotting significant differences. Swapping the problematic device with a known, working one is a

quick way to troubleshoot.

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Isolating the Issue Using Layered ModelsGuidelines for Selecting a Troubleshooting Method

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9.2 Network Troubleshooting

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Troubleshooting ToolsSoftware Troubleshooting ToolsCommon software troubleshooting tools include: NMS tools Knowledge bases Baselining tools Host-based protocol analyzers Cisco IOS EPC

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Troubleshooting ToolsHardware Troubleshooting ToolsCommon hardware troubleshooting tools include: Network analysis module Digital multimeters Cable testers Cable analyzers Portable network analyzers

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Troubleshooting ToolsUsing a Syslog Server for Troubleshooting

Severity Level

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Symptoms and Causes of Network TroubleshootingPhysical Layer Troubleshooting

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Symptoms and Causes of Network TroubleshootingData Link Layer Troubleshooting

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Symptoms and Causes of Network TroubleshootingNetwork Layer Troubleshooting

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Symptoms and Causes of Network TroubleshootingTransport Layer Troubleshooting – ACLs

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Symptoms and Causes of Network TroubleshootingTransport Layer Troubleshooting – NAT for IPv4

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Symptoms and Causes of Network TroubleshootingApplication Layer Troubleshooting

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Troubleshooting IP ConnectivityComponents of Troubleshooting End-to-End ConnectivityWhen there is no end-to-end connectivity, and the administrator chooses to troubleshoot with a bottom-up approach, these are common steps the administrator can take:

Step 1. Check physical connectivity at the point where network communication stops, including cables and hardware. The problem might be with a faulty cable or interface, or involve misconfigured or faulty hardware.

Step 2. Check for duplex mismatches.

Step 3. Check data link and network layer addressing on the local network. This includes IPv4 ARP tables, IPv6 neighbor tables, MAC address tables, and VLAN assignments.

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Troubleshooting IP ConnectivityComponents of Troubleshooting End-to-End Connectivity (cont.)

Step 4. Verify that the default gateway is correct.

Step 5. Ensure that devices are determining the correct path from the source to the destination. Manipulate the routing information if necessary.

Step 6. Verify that the transport layer is functioning properly. Telnet can also be used to test transport layer connections from the command line.

Step 7. Verify that there are no ACLs blocking traffic.

Step 8. Ensure that DNS settings are correct. There should be an accessible DNS server.

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Troubleshooting IP ConnectivityEnd-to-End Connectivity Problem Initiates Troubleshooting

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Troubleshooting IP ConnectivityStep 1. Verify the Physical Layer

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Troubleshooting IP ConnectivityStep 2. Check for Duplex Mismatches

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Troubleshooting IP ConnectivityStep 3. Verify Layer 2 and Layer 3 Addressing on the Local Network

IPv4: arp command (PC) show mac address-table command (router)

IPv6: netsh interface ipv6 show neighbor command (PC)

show ipv6 neighbors (router)

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Troubleshooting IP ConnectivityStep 4. Verify Default Gateway

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Troubleshooting IP ConnectivityStep 5. Verify Correct Path

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Troubleshooting IP ConnectivityStep 6. Verify the Transport Layer

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Troubleshooting IP ConnectivityStep 7. Verify ACLs

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Troubleshooting IP ConnectivityStep 8. Verify DNS

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Chapter 9: Summary

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