Basic Router Troubleshooting
This chapter offers an introduction to network testing and emphasizes the necessity of
using a structured approach to troubleshooting. It also describes the fundamentals of
Basic testing o
f a network should proceed in sequence from one OSI reference model
layer to another. It is best to begin with Layer 1 and work to Layer 7, if necessary.
Beginning with Layer 1, look for simple problems such as power cords that are
unplugged in the wall. T
he most common problems that occur on IP networks result
from errors in the addressing scheme. It is important to test the address configuration
before continuing with further configuration steps.
Each test that is presented in this section focuses on ne
twork operations at a specific
layer of the OSI model.
are just two of the commands that allow for
the testing of a network.
Demonstrate your knowledge of these concepts by answering the following questions in
the space t
hat is provided.
Describe typical Layer 1 errors.
Layer 1 errors can include the following:
Cables connected to the wrong ports
Intermittent cable connections
Cables incorrectly terminated
Wrong cables used for the tas
ks at hand (must use cross
rollovers, and straight
through cables correctly)
DCE cable problems
DTE cable problems
Devices powered off
Describe typical Layer 2 errors.
Layer 2 errors can include the following:
igured serial interfaces
Improperly configured Ethernet interfaces
Incorrect clock rate settings on serial interfaces
Improper encapsulation set on serial interfaces (HDLC is default)
Describe typical Layer 3 errors.
Layer 3 errors can include
Routing protocol not enabled
Wrong routing protocol enabled
Incorrect network/IP addresses
Incorrect subnet masks
Incorrect interface addresses
IP bindings (host table entries)
Wrong autonomous system number for IGRP
Describe some n
etwork troubleshooting strategies.
It’s useful to have a general method to refer to when troubleshooting computer networks.
This section outlines one such method used by many networking professionals.
The steps are as follows:
Define the problem. What are
the symptoms and the potential causes?
Gather the facts. Isolate the possible causes.
Consider the possibilities. Based on the facts gathered, narrow the focus to areas
relevant to the specific problem. This is the step where you set the boundaries for the
Create an action plan. Devise a plan in which you manipulate only
variable at a
Implement the action plan. Perform each step carefully while testing to see whether
the symptom disappears.
Observe the results. Determine whether you resol
ved the problem. If so, the
process is complete.
Repeat the process. If you did not resolve the problem, move to the next most likely
cause on your list. Return to step 4, and repeat the process until you solve the problem.
following terms as completely as you can. Use the online curriculum or
Chapter 18 from the
Cisco Networking Academy Program CCNA 1 and 2
, Third Edition,
Message sent by one network device to inform another network dev
that the virtual circuit between the two is still active.
network interface card. Board that provides network communication
capabilities to and from a computer system.
packet internet groper. ICMP echo message and its reply. Used in IP netwo
to test the connectivity of a network device.
Bridge protocol that uses the spanning
enabling a learning bridge to dynamically work around loops in a network topology by
creating a spanning tree.
dard terminal emulation protocol in the TCP/IP protocol stack. Telnet
is used for remote terminal connection, enabling users to log in to remote systems and
use resources as if they were connected to a local system.
Program available on many sys
tems that traces the path a packet takes
to a destination. It is used mostly to debug routing problems between hosts.
CCNA Exam Review Questions
The following questions help you prepare for the CCNA exam. Answers appear in
Appendix B, "CCNA Exam Review Q
uestions Answer Key.
When a technician performs basic testing of a network, which of the following is true?
The technician should proceed in sequence from one OSI reference model layer
to the next.
The technician should proceed with any desired
The technician should proceed with the management level.
The technician should start at Layer 4 of the OSI model.
When a technician troubleshoots a network, which of the following is true?
The technician should start with a structu
The technician should start with an approach of his choice.
The technician can start with any approach.
The technician should start with client servers.
When a technician troubleshoots a problem on a network, he should do which o
Begin with Layer 1.
Begin with Layer 2.
Begin with Layer 3.
Begin with Layer 4.
If a technician would like to test network connectivity, which basic command should he
When a netwo
rk administrator wants to verify the application layer software between
source and destination stations, which of the following commands should he use?
You suspect that one of the routers that is connected to your n
etwork is sending bad
routing information. Which of the following commands can you use to verify this?
show ip route
show ip route
show ip protocol
show ip protocol
Why would you displ
ay the IP routing table?
To set the router update schedule
To identify destination network addresses and next
To trace where datagrams are coming from
To set the parameters and filters for the router
If you want to see RIP routing
updates as they are sent and received, what command
structure should you use?
show ip rip
debug ip protocols
debug ip rip
show ip rip update
The dynamic output of the
command comes at a performance cos
produces _________ processor overhead.
By default, what does the router send the debug output and system messages to?
command provides what
type of terminal?
What does ICMP stand for?
Internet Control Message Parameter
Internal Control Message Protocol
Internet Control Message Protocol
Internet Control Message Performance
erfaces or NICs will have what type of lights that show whether there is a valid
Telnet is used at what layer of the OSI model?
ting of a network should start at what layer of the OSI reference model?