Cisco network troubleshooting for beginners, more detailed info..

1. Cisco Network Troubleshooting for Beginners
As a Network Engineer, your primary goal is to make sure that your network
equipment is operating properly at all times. But we all know that any equipment can
break down. The reasons for this could be software inconsistencies, hardware
malfunctions, maybe even environmental hazards.
Under such unpredictable conditions, your responsibility is to identify and isolate the
cause of the malfunction and correct it as soon as you can. That’s why it is extremely
helpful to know some specific techniques that have been proven to be crucial and
essential in the networking world.
In today’s article I’ll present the most important commands that you will definitely
find helpful and even mandatory throughout your networking career and specifically
during network troubleshooting situations. The commands I am talking about, and
which are truly invaluable, are:
Ping
Traceroute
Telnet
Show interfaces
Show ip interface
Show ip route
Show running-config
Show startup-config
The Ping Utility
The PING command operates on the Network layer and uses the services of the
ICMP protocol. It is the first command that you should use at the beginning of your
troubleshooting process.
With PING you can test whether a remote host is alive by transmitting echo request
messages and receive echo replies from the specific host. Keep in mind that even if a
host is alive, it does not mean that it is functioning properly that is why PING is used
at the beginning of your investigation and is the best command to start with.
PING command has quite a lot of options from which you can greatly benefit. For
example, you can choose to PING with different network protocols. Moreover, you
are able to set the exact number of PING requests to be transmitted. You can also
choose the length of data carried within the echo request packet (provided in bytes)
and also specify whether fragmentation of this packet is allowed or not during
transmission. The last two options in cooperation can be used to identify the lowest
MTU value existing in the communication path.
On Cisco devices the simpler way to use the PING utility is to issue the command
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2. PING along with the IP address of the remote device:
For more advanced features, simply issue the PING command and follow the
prompts from that point on:
The Traceroute Command
The TRACEROUTE command traces the end-to-end path a packet takes though an
internetwork. Similarly with PING, it uses the ICMP protocol with TTL timeouts to
perform its operation..
This command is very useful in identifying potential link bottlenecks throughout the
transmission path. Here is a sample output of the TRACEROUTE command:
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3. The Telnet Command
Use the telnet command to verify TCP stack and application layer software between
source and destination stations. Of course, to be able to telnet on a Cisco device, the
latter needs to be already configured to accept telnet connections. To use TELNET
just issue the telnet command along with the IP address or hostname of the remote
station:
The Show Interfaces Command
The show interfaces command presents all the available interfaces that can be
configured on your Cisco device. You can explicitly use this command to show only
details on a single interface by issuing the interfaces name after the show interfaces
command. This command is very useful because it can reveal layer 1 and layer 2
problems. Moreover, this command provides details regarding the hardware address
(MAC), IP address, encapsulation method and statistics concerning erroneous
conditions on the specific interface. Examine the output of the show interface
fastethernet 0:
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4. The first line of the show interface command is the most important one. The first
parameter refers to the physical layer, thereforein this case FastEthernet 0 is up and
operating. The second part of the line refers to the Data Link Layer; therefore here
line protocol is up means that we have layer 2 connectivity as well. There are four
possible outputs that you may come across:
FastEthernet0 is up, line protocol is up: Both the Physical and Data Link layers
on the interface are functioning correctly.
FastEthernet0 is down, line protocol is down: This output indicates a physical
interface problem. For example, the cable on this interface or on the remote
interface is disconnected.
FastEthernet0 is up, line protocol is down: In this case, Physical layer is
operational. The line protocol being down indicates a clocking or framing
problem. Probable reasons for this are encapsulation and clock rate
mismatches.
Ethernet0 is administratively down, line protocol is down: This output
indicates that a local interface has been manually shut down using the
shutdown command.
The Show IP Interface Command
The show ip interface command will provide details regarding layer 3 configuration
on the interfaces. Using this command you can see the IP address and mask
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5. configured on a given interface, whether an access list is applied on the interface as
well as basic
IP information.
The Show IP Route Command
Use the show ip route command to find detailed information regarding the routes
configured on the router. Keep in mind that the router can only route packets to the
networks listed in its routing table. It is possible that a router cannot reach a network
you manually configured therefore, that specific route is removed from its table and
that is why you should use the show running configuration command in conjunction
with show ip route to spot possible inconsistencies.
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6. Show Running-Config and Show Startup-Config Commands
Issue the show running configuration command to find out the whole configuration
your Cisco devices use while operating. By looking at the details of your devices,
configuration can help you identify the cause to your problem. Use the show startup
configuration command to see the configuration commands that will be loaded to
your device the next time it reboots and compare it with your running configuration
in order to identify possible unconformities.
In the End … Follow Your Instincts
Isolating a network malfunction is not an easy task. In many cases you need to swim
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7. into really deep waters in order to identify the error. Meaning, that you will probably
need to get into the bits and bytes in order to locate the error.
A lot of network monitor tools are available to help you during these difficult tasks.
And although there’s a lot you can do, remember to never lose your courage; have
patience and follow your gut.
Sometimes the problem that you’re searching for will be right in front of you and will
slip out of your hands, just when you think you have it. So be persistent, be patient
and remember: you will win the battle!
Be prepared; in order to fight well, you should be armed well. So get to know your
device very well, know how it operates, and know how to use the basic
troubleshooting commands – that you just learned – so that you can identify the
problem, or at least estimate where the problem might be. Good luck!
More Networking Info and Tips:
CCNP TSHOOT: Cisco Troubleshooting Techniques & Procedures
How to Troubleshoot OSPF?
How to Configure EIGRP on a Cisco Router?
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