Quản trị mạng - Chapter 8: Network troubleshooting
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.
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1Chapter 8 –
Network Troubleshooting
CCNA Exploration 4.0
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Notation of the CCNA Voucher
• Testking CCNA 640-802
• Pass4sure
• Testinside
• Actualtest
• Vnpro.org
• Sadikhov.com
•
• 9tut.net
• Examcollection.com
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Introduction
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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
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
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Documenting Your Network
1. Network Configuration Table
The 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
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Documenting Your Network
2. End-System Documentation
The 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
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Documenting Your Network
3. Network Topology Diagram
At a minimum, the topology diagram should include:
– Symbols for all devices and how they are connected
– Interface types and numbers
– IP addresses
– Subnet masks
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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
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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.
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Steps for Establishing a Network Baseline
• Animation
8.1.4.1
• Step 1.
Determine what
types of data to
collect
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Steps for Establishing a Network Baseline
• Step 2. Identify devices and ports of interest
Devices and ports of interest include:
– Network device ports that connect to other network devices
– Servers
– Key users
– Anything else considered critical to operations.
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Steps for Establishing a Network Baseline
• 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. 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.
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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.
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Measuring Network Performance Data
• In simpler networks, the baseline tasks may require a combination of
manual data collection and simple network protocol inspectors.
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Troubleshooting Methodologies
and Tools
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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.
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• Using Layered
Models for
Troubleshooting
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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.
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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.
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Troubleshooting Methods
• 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.
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Troubleshooting Methods
• 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.
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Guidelines for Selecting a Troubleshooting Method
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Gathering Symptoms
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Gathering Symptoms
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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.
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Software Troubleshooting Tools
• 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
This is a free tool providing information on Cisco-
related hardware and software.
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Software Troubleshooting Tools
• 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.
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Software Troubleshooting Tools
• 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.
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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.
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Hardware Troubleshooting Tools
• 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.
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Hardware Troubleshooting Tools
• 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.
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Troubleshooting Tools
Refer to 8.2.6.3
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Common WAN
Implementation Issues
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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.
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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.
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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.
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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.
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WAN Topology Considerations
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WAN Topology Considerations
• 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.
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WAN Connection Technologies
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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.
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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.
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Common WAN Implementation Issues
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Case Study: WAN Troubleshooting from an ISP’s 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
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Network Troubleshooting
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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
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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
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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.
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Physical Layer Troubleshooting
• 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
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Physical Layer Troubleshooting
• 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.
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Data Link Layer Troubleshooting
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Data Link Layer Troubleshooting
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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.
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Troubleshooting Layer 2 - PPP
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Troubleshooting Layer 2 - Frame Relay
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Troubleshooting Layer 2 - Frame Relay
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Troubleshooting Layer 2 - Frame Relay
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Troubleshooting Layer 2 - Frame Relay
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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.
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Network Layer Troubleshooting
• Network layer problems include any problem that involves a Layer 3
protocol, both routed protocols and routing protocols.
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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.
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Transport Layer Troubleshooting
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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
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Application Layer Troubleshooting
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Application Layer Troubleshooting
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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.
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Application Layer Troubleshooting
• 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.
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Application Layer Troubleshooting
• 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.
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Labs
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Summary
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