Chapter 08 network troubleshooting

Chapter 8: Network Troubleshooting
CCNA Exploration 4.0
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Objectives
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Establish and document a network baseline.
•
Describe the various troubleshooting methodologies and
troubleshooting tools.
•
Describe the common issues that occur during WAN
implementation.
•
Identify and troubleshoot common enterprise network
implementation issues using a layered model approach.
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Establishing the
Network Performance Baseline
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Documenting Your Network
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To efficiently diagnose and correct network problems, a
network engineer needs to know how a network has been
designed and what the expected performance for this
network should be under normal operating conditions. This
information is called the network baseline.
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Network documentation should include these components:
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Network configuration table
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End-system configuration table
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Network topology diagram
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Network configuration table
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Contains accurate, up-to-date records of the hardware and
software used in a network.
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End-system Configuration Table
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Contains baseline records of the hardware and software
used in end-system devices such as servers, network
management consoles, and desktop workstations.
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Network Topology Diagram
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Graphical representation of a network, which illustrates how
each device in a network is connected and its logical
architecture.
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Documenting Your Network
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Network Documentation Process
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When you document your network, you may have to gather
information directly from routers and switches. Commands
that are useful to the network documentation process
include:
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The ping command
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The telnet command
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The show ip interface brief command
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The show ip route command
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The show cdp neighbor detail command
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Documenting Your Network
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Why Is Establishing a Network Baseline Important?
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Steps for Establishing a Network Baseline
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Planning for the First Basline
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Step 1. Determine what types of data to collect
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Steps for Establishing a Network Baseline
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Step 2. Identify devices and ports of interest
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Steps for Establishing a Network Baseline
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Step 3. Determine the baseline duration
It is important that the length of time and the baseline
information being gathered are sufficient to establish a
typical picture of the network.

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Steps for Establishing a Network Baseline
Measuring Network Performance Data
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Automated Data Collection: Fluke Network SuperAgent
module
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Steps for Establishing a Network Baseline
Measuring Network Performance Data
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Manual Commands
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Troubleshooting Methodologies and
Tools
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A General Approach to Troubleshooting
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Network engineers, administrators, and support personnel
realize that troubleshooting is a process that takes the
greatest percentage their time.
•
Two extreme approaches to troubleshooting almost always
result in disappointment, delay, or failure.
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The theorist, or rocket scientist, approach
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The impractical, or caveman, approach.
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The rocket scientist: analyzes and
reanalyzes the situation until the exact

cause at the root of the problem has
been identified and corrected with
surgical precision.
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A General Approach to Troubleshooting
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The caveman: first instinct is to start
swapping cards, cables, hardware, and
software until miraculously the network
begins operating again.
•
Since both of these approaches are
extremes, the better approach is
somewhere in the middle using elements
of both. It is important to analyze the
network as a whole rather than in a
piecemeal fashion. A systematic
approach minimizes confusion and cuts
down on time otherwise wasted with trial
and error.
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Using Layered Models for Troubleshooting
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OSI Versus TCP/IP Layered Models
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General Troubleshooting Procedures
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General troubleshooting process:
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Troubleshooting Methods

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There are three main methods for troubleshooting networks:
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Bottom up
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Top down
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Divide and conquer
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Bottom-Up Troubleshooting Method
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Good approach to use when the
problem is suspected to be a
physical one.
–
Disadvantage: requires checking
every device and interface
until the possible cause of the
problem is found.
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Troubleshooting Methods
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Top-Down Troubleshooting Method
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Use for simpler problems or when
you think the problem is with a piece
of software.
–
Disadvantage: requires checking
every network application until the

possible cause of the problem is
found.
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Divide-and-Conquer Troubleshooting
Method
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You select a layer and test in both
directions from the starting layer.
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Troubleshooting Methods
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Guidelines for Selecting a Troubleshooting Method
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Gathering Symptoms
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Step 1. Analyze existing
symptoms
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Step 2. Determine
ownership
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Step 3. Narrow the scope
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Step 4. Gather symptoms
from suspect devices
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Step 5. Document
symptoms