Chap 8 – Network Troubleshooting
Learning Objectives
•Establish a network baseline
•Describe troubleshooting methodologies and
troubleshooting tools
•Describe the common issues that occur during
WAN implementation
•Troubleshoot enterprise network implementation
issues
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Chapter 8
Network Documentation
Network Configuration Documentation
End-System Documentation
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Chapter 8
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:
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The ping command is used to test connectivity with neighboring devices
before logging in to them. Pinging to other PCs in the network also
initiates the MAC address auto-discovery process.
The telnet command is used to log in remotely to a device for accessing
configuration information.
The show ip interface brief command is used to display the up or down
status and IP address of all interfaces on a device.
The show ip route command is used to display the routing table in a
router to learn the directly connected neighbors, more remote devices
(through learned routes), and the routing protocols that have been
configured.
The show cdp neighbor detail command is used to obtain detailed
information about directly connected Cisco neighbor devices.
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Chapter 8
Establish a Network Baseline
A Network Baseline helps to determine the
"personality" of the network and provides answers to
the following questions:
1. How does the network perform during a normal or
average day?
2. Where are the under-utilised and over-utilised
areas?
3. Where are the most errors occurring?
4. What thresholds should be set for the devices that
need to be monitored?
5. Can the network deliver the identified policies?
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Chapter 8
Establishing a Baseline
1. Determine what types of data to collect
2. Identify devices and ports of interest
3. Determine the baseline duration
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Chapter 8
Establishing a Baseline
Sophisticated network management software is
often used to baseline large and complex
networks.
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Chapter 8
Systematic Troubleshooting
•It is important to analyse the network as a
whole rather than in a piecemeal fashion.
•A systematic approach minimises confusion and
cuts down on time otherwise wasted with trial
and error.
•Logical networking models, such as the OSI and
TCP/IP models, separate network functionality
into modular layers. When troubleshooting,
these layered models can be applied to the
physical network to isolate network problems.
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Chapter 8
Troubleshooting Methodologies and
Tools
Stage 1
Stage 2
Stage 3
Gather
Symptoms
Isolate
Problem
Correct
Problem
Retest –
Further
Problems?
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Chapter 8
Gathering Symptoms
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Step 1. Analyse existing symptoms – Read fault report and question
user to understand problem.
Step 2. Determine ownership – Is the problem within your system, or
those of an external agency?
Step 3. Narrow the scope - Determine if the problem is at the core,
distribution, or access layer of the network. At the identified layer,
analyse the existing symptoms and use your knowledge of the
network topology to determine which pieces of equipment are the
most likely cause.
Step 4. Gather symptoms from suspect devices - Using a layered
troubleshooting approach, gather hardware and software symptoms
from the suspect devices.
Step 5. Document symptoms - Sometimes the problem can be solved
using the documented symptoms.
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Chapter 8
Gathering Symptoms
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Chapter 8
Software Troubleshooting
Tools
•Network management system (NMS) tools include
device-level monitoring, configuration, and fault
management tools.
•On-line network device vendor knowledge bases
have become indispensable sources of information.
•A protocol analyser decodes the various protocol
layers in a recorded frame and presents this
information in a relatively easy to use format.
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Chapter 8
Hardware Troubleshooting
Tools
Digital
Multimeter
(DMM)
Cable
Tester
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Cable
Analyser
Network
Analyser
Chapter 8
Steps In WAN Design
Locate LANs
Analyse Traffic
Plan Topology
Plan Bandwidth
Choose Technology
Cost & Evaluate
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Where are they? How many users?
Video? Data? Voice? Latency/Jitter
Circuit? Packet? Cell?
UTP? Fibre?
Switches? Routers?
Can you afford it?
Chapter 8
WAN Traffic Analysis
•Determine what data traffic must be carried, its origin,
and its destination.
•WANs carry a variety of traffic types with varying
requirements for bandwidth, latency, and jitter:
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Chapter 8
Plan the WAN Topology
•When many locations must be joined, a hierarchical
solution is recommended.
•LANs can be grouped into
areas and interconnected
them to form a region, which
are in turn interconnected to
form the core of the WAN.
•The area would have a star
topology, with the hubs of the
stars linked to form the
region.
•Regions could be geographic,
connecting between three and
10 areas, and the hub of each
region could be linked pointto-point.
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Chapter 8
WAN Technology Bandwidth
•A typical private WAN uses a combination of
technologies that are usually chosen based on
traffic type and volume.
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Chapter 8
Symptoms of Physical Layer Problems
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Performance Lower than
baseline
Loss of connectivity
High collisions
Network bottlenecks or
congestion
Nigh CPU utilisation
rates
Console error messages
Chapter 8
Causes of Physical Layer Problems
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Power related problems
Hardware faults
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Cabling faults
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Attenuation
Noise (EMI)
Interface configuration
errors
Exceeding design limits
CPU overload
Chapter 8
Physical Layer Troubleshooting
Check for bad cables or
connections
Check that correct cable
standard has been used
Check if devices have been
cabled correctly
Cable
Analyser
Verify correct interface
configuration
Check operating statistics
and data error rates
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Chapter 8
Physical Layer Troubleshooting
show interface serial ##
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Chapter 8
Physical Layer Troubleshooting
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Chapter 8
Symptoms of Data Link Layer
Problems
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No connectivity at the
network layer or above
No functionality at the
network layer or above
Network performance
below baseline
Excessive broadcasts
Console error messages
Hardware faults
Chapter 8
Causes of Data Link Layer
Problems
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Encapsulation errors
Address mapping errors
Framing errors
STP failures or loops
Chapter 8
Data Link Layer Troubleshooting
Check encapsulation
settings
Check authentication
settings
Check LMI configuration
(frame relay)
Check PVC status
(frame relay)
Check DLCI mapping
(frame relay)
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Chapter 8
Data Link Layer Troubleshooting
1.
Identify that an STP loop is occurring - Loss of connectivity,
high CPU utilization on routers connected to affected segments
or VLANs , high link utilization (often 100 percent)
2. Discover the scope of the loop - highest priority is to stop the
loop and restore network operation by discovering which ports
are involved.
3. Break the loop - shut down or disconnect the involved ports one
at a time.
4. Find and fix the cause of the loop - investigate the topology
diagram to find a redundant path.
5. Restore the redundancy - after the device or link that is causing
the loop has been found and the problem has been resolved,
restore the redundant links that were disconnected.
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Chapter 8
Symptoms of Network Layer
Problems
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Network failure
Network performance
below baseline
Chapter 8
Causes of Network Layer
Problems
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Topology changes
Connectivity problems
Neighbor issues
Routing issue
Chapter 8
Network Layer Troubleshooting
Check for network
topology changes
Check for equipment &
connectivity problems
Check routing neighbor
relationships
Check topology tables
Check routing tables
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Chapter 8
Symptoms of Transport Layer
Problems
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Intermittent network
problems
Security problems
Address translation
problems
Problems with specific
traffic types
Chapter 8
Causes of Transport Layer ACL
Problems
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Applied to incorrect traffic
Incorrect order
Implicit deny any any
Addresses & wildcard masks
TCP / UDP selection
Source & Destination ports
Use of established keyword
Uncommon protocols
Chapter 8
Causes of Transport Layer NAT
Problems
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Interoperability issues
Incorrect static NAT
Incorrect NAT timers
Chapter 8
Symptoms of Application Layer
Problems
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User complaints about
slow performance
Console error messages
System log file messages
Network management
system alarms
Chapter 8
Application Layer Troubleshooting
Ping the default gateway
Verify end-to-end
connectivity
Verify ACL and NAT
operation
Troubleshoot upper
layer protocol
connectivity
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Chapter 8
Chap 8 – Network Troubleshooting
Learning Objectives
•Establish a network baseline
•Describe troubleshooting methodologies and
troubleshooting tools
•Describe the common issues that occur during
WAN implementation
•Troubleshoot enterprise network implementation
issues
34
Chapter 8
Any
Questions?
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Chapter 8