Chapter 08 network troubleshooting

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Chapter 8: Network Troubleshooting

CCNA Exploration 4.0

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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

• 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

• Network documentation should include these components:

– Network configuration table

– End-system configuration table

– Network topology diagram

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Network configuration table

• Contains accurate, up-to-date records of the hardware and software used in a network

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End-system Configuration Table

• Contains baseline records of the hardware and software

used in end-system devices such as servers, network

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Network Topology Diagram

• Graphical representation of a network, which illustrates how each device in a network is connected and its logical

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Network Documentation Process

• 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:

– The ping command

– The telnet command

– The show ip interface brief command

– The show ip route command

– The show cdp neighbor detail command

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Why Is Establishing a Network Baseline Important?

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Steps for Establishing a Network Baseline

• Planning for the First Basline

• Step 1 Determine what types of data to collect

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• Step 2 Identify devices and ports of interest

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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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Measuring Network Performance Data

• Automated Data Collection: Fluke Network SuperAgent

module

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Measuring Network Performance Data

• Manual Commands

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Troubleshooting Methodologies and

Tools

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A General Approach to Troubleshooting

• 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

– The theorist, or rocket scientist, approach

– The impractical, or caveman, approach.

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

• 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

• OSI Versus TCP/IP Layered Models

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General Troubleshooting Procedures

• General troubleshooting process:

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Troubleshooting Methods

• There are three main methods for troubleshooting networks:

– Bottom up

– Top down

– Divide and conquer

• Bottom-Up Troubleshooting Method

– 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

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• Top-Down Troubleshooting Method

– 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

• Divide-and-Conquer Troubleshooting

Method

– You select a layer and test in both

directions from the starting layer

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• Guidelines for Selecting a Troubleshooting Method

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• Step 3 Narrow the scope

• Step 4 Gather symptoms

from suspect devices

• Step 5 Document

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Gathering Symptoms

• Use the Cisco IOS commands to gather symptoms about the network

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Gathering Symptoms

• Questioning End Users

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Software Troubleshooting Tools

• NMS Tools

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• Knowledge Bases

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• Baselining Tools

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• Protocol Analyzers

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Hardware Troubleshooting Tools

• Network Analysis Module (NAM)

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Digital Multimeters Cable Testers

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Cable Analyzers Portable Network Analyzers

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http://www.flukenetworks.com/fnet/en-Common WAN Implementation Issues

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slower than LAN bandwidth

• WANs carry a variety of traffic types, such as data, voice,

and video

• Older WANs often consisted of data links directly connecting remote mainframe computers

• Today's WANs connect geographically separated LANs

WAN technologies function at the lower three layers of the

OSI reference model

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Steps in WAN Design

COST AND EVALUATE

Iterate Review

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WAN Traffic Considerations

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WAN Topology Considerations

Star or Hub-and-Spoke Full-Mesh

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WAN Connection Technologies

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WAN Topology Considerations

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WAN Bandwidth Considerations

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Common WAN Implement Issues

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Case Study: WAN Troubleshooting from a ISP’s

Perspective

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Network Troubleshooting

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Interpreting Network Diagrams to Identify Problems

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Interpreting Network Diagrams to Identify Problems

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Physical Layer Troubleshooting

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• Troubleshooting

Layer 1 Problems

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Data Link Layer Troubleshooting

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Data Link Layer Troubleshooting

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Troubleshooting Layer 2 - PPP

• Step 1: Check that the appropriate encapsulation is in use at both ends

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• Step 2: Confirm that the Link Control Protocol (LCP)

negotiations have succeeded

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• Step 3: Verify authentication on both sides of the link

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Troubleshooting Layer 2 – Frame Relay

• Step 1: Verify the physical connection between the

CSU/DSU and the router

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• Step 2: Verify proper LMI information exchange between

each router and the FR switch

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• Step 3: Verify that PVC status is active

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• Step 4: Verify that Frame Relay encapsulation matches on

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Troubleshooting Layer 2 - STP Loops

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Network Layer Troubleshooting

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Network Layer Troubleshooting

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Transport Layer Troubleshooting

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Application Layer Troubleshooting

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In this chapter, you have learned to: