Cisco Network Troubleshooting for Beginners
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January 7, 2008

Cisco Network Troubleshooting for Beginners

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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.

I’ve covered PING extensively in one of my previous posts, so for more details on PING, check out: How to Troubleshoot Your Connections with Ping and Traceroute.

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 PING along with the IP address of the remote device:

Network Troubleshooting 1

For more advanced features, simply issue the PING command and follow the prompts from that point on:

Network Troubleshooting 2


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.

Again, for more details on this command see How to Troubleshoot Your Connections with Ping and Traceroute.

This command is very useful in identifying potential link bottlenecks throughout the transmission path. Here is a sample output of the TRACEROUTE command:

Network Troubleshooting 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:

Network Troubleshooting 4


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:

Network Troubleshooting 5

The first line of the show interface command is the most important one. The first parameter refers to the physical layer, therefore
in 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 configured on a given interface, whether an access list is applied on the interface as well as basic
IP information.

Network Troubleshooting 6


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 can not 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.

For more details on IP routing issues see my article on Default and Static Routing Basics.

Network Troubleshooting 7


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.

Network Troubleshooting 8


In the End … Follow Your Instincts

Isolating a network malfunction is not an easy task. In many cases you need to swim 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!

 


 

About the Author

(CCNA, NET+, MOUS) holds a BSc in Electronic Engineering and an MSc in Communication Networks. He has over three years of experience in teaching MS Office applications, networking courses and GCE courses in Information Technology. Stelios is currently working as a VoIP Engineer in a Telecom company, where he uses his knowledge in practice. He has successfully completed training on CCNP topics, Linux and IMS. His enthusiasm, ambition and knowledge motivate him to offer his best. Stelios has written many articles covering Cisco CCENT, CCNA, and CCNP.


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