CCNA 3 - Troubleshooting the network

© 2008 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 1
Chapter 9:
Troubleshooting the
Network
Connecting Networks

Presentation_ID 2© 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential
Chapter 9
9.0 Introduction
9.1 Troubleshooting with a Systematic Approach
9.2 Network Troubleshooting
9.3 Summary

9.1 Troubleshooting with a
Systematic Approach

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.

Network Documentation
Documenting the Network
Network 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

Network Topology Diagrams
 Physical Topology
Device type
Model and manufacturer
Operating system version
Cable type and identifier
Cable specification
Connector type
Cabling endpoints
 Logical Topology
Device identifiers
IP address and prefix lengths
Interface identifiers
Connection type
DLCI for virtual circuits
Site-to-site VPNs
Routing protocols
Static routes
Data-link protocols
WAN technologies used

Establishing a Network Baseline

Establishing 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.

Measuring 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

Troubleshooting Process
General Troubleshooting Procedures

Gathering Symptoms

Questioning End Users

Isolating the Issue Using Layered Models
Using Layered Models for Troubleshooting

Troubleshooting Methods
Using the layered models, there are three primary methods for
troubleshooting networks:
 Bottom-up
 Top-down
 Divide-and-conquer

Troubleshooting 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.

Guidelines for Selecting a Troubleshooting Method

9.2 Network Troubleshooting

Troubleshooting Tools
Software Troubleshooting Tools
Common software troubleshooting tools include:
 NMS tools
 Knowledge bases
 Baselining tools
 Host-based protocol analyzers
 Cisco IOS EPC

Hardware Troubleshooting Tools
Common hardware troubleshooting tools include:
 Network analysis module
 Digital multimeters
 Cable testers
 Cable analyzers
 Portable network analyzers

Using a Syslog Server for Troubleshooting
Severity Level

Symptoms and Causes of Network Troubleshooting
Physical Layer Troubleshooting

Data Link Layer Troubleshooting

Network Layer Troubleshooting

Transport Layer Troubleshooting – ACLs

Transport Layer Troubleshooting – NAT for IPv4

Application Layer Troubleshooting

Troubleshooting IP Connectivity
Components of Troubleshooting End-to-End Connectivity
When 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.

Components 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.

End-to-End Connectivity Problem Initiates Troubleshooting

Step 1. Verify the Physical Layer

Step 2. Check for Duplex Mismatches

Step 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)

Step 4. Verify Default Gateway

Step 5. Verify Correct Path

Step 6. Verify the Transport Layer

Step 7. Verify ACLs

Step 8. Verify DNS

Chapter 9: Summary

CCNA 3 - Troubleshooting the network

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