ccna exp4 - chapter08 - network troubleshooting

1Chapter 8 Network Troubleshooting

CCNA Exploration 4.0

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Introduction


Establishing the Network

Performance Baseline


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

and is captured in documentation such as configuration tables and

topology diagrams.

Network documentation should include these components: 1.Network configuration table

2.End-system configuration table

3.Network topology diagram



1. Network Configuration TableThe data set that should be included for all components:

Type of device, model designation IOS image name Device network hostname Location of the device (building, floor, room, rack, panel) If it is a modular device, include all module types and in which module slot they are

located

Data link layer addresses Network layer addresses Any additional important information about physical aspects of the device



2. End-System DocumentationThe following information should be documented:

Device name (purpose)

Operating system and version

IP address

Subnet mask

Default gateway, DNS server, and WINS server addresses

Any high-bandwidth network applications that the end-system runs



3. Network Topology DiagramAt a minimum, the topology diagram should include:

Symbols for all devices and how they are connected Interface types and numbers IP addresses Subnet masks


Network Documentation Process

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


Why is Establishing a Network Baseline Important ?

Establishing a network performance baseline requires collecting key performance data from the ports and devices that are essential to network operation.


Steps for Establishing a Network Baseline

Animation 8.1.4.1

Step 1. Determine what

types of data to

collect



Step 2. Identify devices and ports of interestDevices and ports of interest include:

Network device ports that connect to other network devices Servers Key users Anything else considered critical to operations.



Step 3. Determine the baseline durationIt is important that the length of time and the baseline information being

gathered are sufficient to establish a typical picture of the network. This

period should be at least seven days to capture any daily or weekly

trends. Weekly trends are just as important as daily or hourly trends.


Measuring Network Performance Data

Sophisticated network management software is often used to baseline large and complex networks.

For example, the Fluke Network SuperAgent module enables administrators to automatically create and review reports using its Intelligent Baselines feature.


Measuring Network Performance Data

In simpler networks, the baseline tasks may require a combination of manual data collection and simple network protocol inspectors.


Troubleshooting Methodologies

and Tools


A General Approach to Troubleshooting

Two extreme approaches to troubleshooting almost always result in disappointment, delay, or failure. At one extreme is the theorist, or rocket scientist, approach. At the other extreme is the impractical, or caveman, approach.

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.

The caveman's first instinct is to start swapping cards, cables, hardware, and software until miraculously the network begins operating again.


Using Layered Models for

Troubleshooting


General Troubleshooting Procedures

Stage 1 Gather symptoms - Troubleshooting begins with the process of gathering and documenting symptoms from the network, end systems, and users.

Stage 2 Isolate the problem - The problem is not truly isolated until a single problem, or a set of related problems, is identified.

Stage 3 Correct the problem - Having isolated and identified the cause of the problem, the network administrator works to correct the problem by implementing, testing, and documenting a solution.


Troubleshooting Methods

There are three main methods for troubleshooting networks:1. Bottom up

2. Top down

3. Divide and conquer

Bottom-Up Troubleshooting Method In bottom-up troubleshooting you start with the physical components of the network

and move up through the layers of the OSI model until the cause of the problem is identified.



Top-Down Troubleshooting Method In top-down troubleshooting your start with the end-user applications and

move down through the layers of the OSI model until the cause of the problem has been identified.



Divide-and-Conquer Troubleshooting Method When you apply the divide-and-conquer approach toward troubleshooting

a networking problem, you select a layer and test in both directions from

the starting layer.


Guidelines for Selecting a Troubleshooting Method


Gathering Symptoms


Software Troubleshooting Tools

NMS Tools Network management system (NMS) tools include device-level

monitoring, configuration, and fault management tools.

Examples of commonly used network management tools are CiscoView, HP Openview, Solar Winds, and What's Up Gold.



Knowledge Bases On-line network device vendor knowledge bases have become

indispensable sources of information.

The figure shows the Cisco Tools & Resources page found at http://www.cisco.com. This is a free tool providing information on Cisco-related hardware and software.



Baselining Tools Many tools for automating the network documentation and baselining

process are available. These tools are available for Windows, Linux, AUX operating systems.

The figure shows a screen chapter of the SolarWinds LANsurveyor and CyberGauge software.



Protocol Analyzers A protocol analyzer decodes the various protocol layers in a recorded

frame and presents this information in a relatively easy to use format.

The figure shows a screen capture of the Wireshark protocol analyzer.


Hardware Troubleshooting Tools

Network Analysis Module A network analysis module (NAM) can be installed in Cisco

Catalyst 6500 series switches and Cisco 7600 series routers to provide a graphical representation of traffic from local and remote switches and routers.



Digital Multimeters Digital multimeters (DMMs) are test instruments that are used to directly measure

electrical values of voltage, current, and resistance.

Cable Testers Cable testers are specialized, handheld devices designed for testing the various

types of data communication cabling.



Cable Analyzers Cable analyzers are multifunctional handheld devices that are used to test and

certify copper and fiber cables for different services and standards.

Portable Network Analyzers Portable devices that are used for troubleshooting switched networks and VLANs.


Troubleshooting Tools

Refer to 8.2.6.3


Common WAN

Implementation Issues


WAN Communications

A communications provider or a common carrier normally owns the data links that make up a WAN. The links are made available to subscribers for a fee and are used to interconnect LANs or connect to remote networks.

WAN data transfer speed (bandwidth) is considerably slower than the common LAN bandwidth.

The charges for link provision are the major cost element, therefore the WAN implementation must aim to provide maximum bandwidth at acceptable cost.


Steps in WAN Design

Businesses install WAN connectivity to meet the strategic business requirement of moving data between external branches.

Because WAN connectivity is important to the business and expensive, you need to design the WAN in a systematic manner.


WAN Traffic Considerations

To determine traffic flow conditions and timing of a WAN link, you need to analyze the traffic characteristics specific to each LAN that is connected to the WAN.


WAN Topology Considerations

Many WANs use a star topology. As the enterprise grows and new branches are added, the branches are connected back to the head office, producing a traditional star topology.

Star endpoints are sometimes cross-connected, creating a mesh or partial mesh topology. This provides for many possible combinations for interconnections.



A three-layer hierarchy is often useful when the network traffic mirrors the enterprise branch structure and is divided into regions, areas, and branches.

It is also useful when there is a central service to which all branches must have access but traffic levels are insufficient to justify direct connection of a branch to the service.


WAN Connection Technologies


Using the Internet as a WAN

Although the Internet may pose a security problem it does provides an alternative for inter-branch traffic.

Part of the traffic that must be considered during design is going to or coming from the Internet.


WAN Bandwidth Considerations

Many companies rely on the high-speed transfer of data between remote locations. Consequently, higher bandwidth is crucial because it allows more data to be transmitted in a given time.

When bandwidth is inadequate, competition between various types of traffic causes response times to increase, which reduces employee productivity and slows down critical web-based business processes.


Common WAN Implementation Issues


Case Study: WAN Troubleshooting from an ISPs Perspective

A significant proportion of the support calls

received by an ISP refer

to slowness of the

Network.

To troubleshoot this effectively, you have to

isolate the individual

components and test

each one as follows:

Individual PC host LAN Link from the edge of the user network to the edge of the ISP Backbone of the ISP Server being accessed


Network Troubleshooting


Interpreting Network Diagrams to Identify Problems

Information recorded on the physical network diagram typically includes:

Device type

Model and manufacturer

Operating system version

Cable type and identifier

Cable specification

Connector type

Cabling endpoints


Interpreting Network Diagrams to Identify Problems

Information recorded on a logical network diagram may include:

Device identifiers IP address and

subnet

Interface identifiers Connection type DLCI for virtual

circuits

Site-to-site VPNs Routing protocols Static routes Data-link protocols WAN technologies

used


Physical Layer Troubleshooting

A physical layer problem occurs when the physical properties of the connection are substandard, causing data to be transferred at a rate that is consistently less than the rate of data flow established in the baseline.



Examples of configuration errors that affect the physical layer include: Serial links reconfigured as asynchronous instead of synchronous

Incorrect clock rate

Incorrect clock source

Interface not turned on



Check operational statistics and data error rates Use Cisco show commands to check for statistics such as

collisions and input and output errors. The characteristics of these statistics vary depending on the protocols used on the network.


Data Link Layer Troubleshooting


Troubleshooting Layer 2 - PPP

The difficulty in troubleshooting Layer 2 technologies, such as PPP and Frame Relay, is the unavailability of common Layer 3 troubleshooting

tools, such as ping, to assist with anything but the identification that the

network is down.

Most of the problems that occur with PPP involve link negotiation.


Troubleshooting Layer 2 - PPP


Troubleshooting Layer 2 - Frame Relay


Troubleshooting Layer 2 - STP Loops

To verify STP operation, use the show spanning-tree command on each switch. If you discover that STP is not operating, you can enable it using the spanning-tree vlan

ID command.


Network Layer Troubleshooting

Network layer problems include any problem that involves a Layer 3 protocol, both routed protocols and routing protocols.


Network Layer Troubleshooting

In most networks, static routes are used in combination with dynamic routing protocols.

Improper configuration of static routes can lead to less than optimal routing and, in some cases, create routing loops or parts of the network to become unreachable.

Troubleshooting dynamic routing protocols requires a thorough understanding of how the specific routing protocol functions.


Transport Layer Troubleshooting


Transport Layer Troubleshooting

The biggest problem with all NAT technologies is interoperability with other network technologies, especially those that contain or derive information from host network addressing in the packet.

Some of these technologies include: BOOTP and DHCP DNS and WINS SNMP Tunneling and encryption protocols


Application Layer Troubleshooting



A problem at the application layer can result in unreachable or unusable resources when the physical, data link, network, and transport layers are functional.

Another type of problem at the application layer occurs when the physical, data link, network, and transport layers are functional, but the data transfer and requests for network services from a single network service or application do not meet the normal expectations of a user.



The same general troubleshooting process that is used to isolate problems at the lower layers can also be used to isolate problems at the application layer.

The concepts are the same, but the technological focus has shifted to involve things such as refused or timed out connections, access lists, and DNS issues.



Step 1: Make a backup Step 2: Make an initial

hardware or software configuration change.

Step 3: Evaluate and document each change and its results.

Step 4: Determine if the change solves the problem.

Step 5: Stop when the problem is solved.

Step 6: If necessary, get assistance from outside resources.

Step 7: Document.


Labs


Summary

ccna exp4 - chapter08 - network troubleshooting

Documents

network engineer

network operation

network baselineand

network baseline important

network device ports

network baseline step

network baseline animation

network configuration