Cisco network troubleshooting for beginners


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Cisco network troubleshooting for beginners

  1. 1. Cisco Network Troubleshooting for BeginnersAs a Network Engineer, your primary goal is to make sure that your networkequipment is operating properly at all times. But we all know that any equipment canbreak down. The reasons for this could be software inconsistencies, hardwaremalfunctions, maybe even environmental hazards.Under such unpredictable conditions, your responsibility is to identify and isolate thecause of the malfunction and correct it as soon as you can. That’s why it is extremelyhelpful to know some specific techniques that have been proven to be crucial andessential in the networking world.In today’s article I’ll present the most important commands that you will definitelyfind helpful and even mandatory throughout your networking career and specificallyduring network troubleshooting situations. The commands I am talking about, andwhich are truly invaluable, are: Ping Traceroute Telnet Show interfaces Show ip interface Show ip route Show running-config Show startup-configThe Ping UtilityThe PING command operates on the Network layer and uses the services of theICMP protocol. It is the first command that you should use at the beginning of yourtroubleshooting process.With PING you can test whether a remote host is alive by transmitting echo requestmessages and receive echo replies from the specific host. Keep in mind that even if ahost is alive, it does not mean that it is functioning properly that is why PING is usedat 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. Forexample, you can choose to PING with different network protocols. Moreover, youare able to set the exact number of PING requests to be transmitted. You can alsochoose 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 duringtransmission. The last two options in cooperation can be used to identify the lowestMTU value existing in the communication path.On Cisco devices the simpler way to use the PING utility is to issue the command
  2. 2. PING along with the IP address of the remote device:For more advanced features, simply issue the PING command and follow theprompts from that point on:The Traceroute CommandThe TRACEROUTE command traces the end-to-end path a packet takes though aninternetwork. Similarly with PING, it uses the ICMP protocol with TTL timeouts toperform its operation..This command is very useful in identifying potential link bottlenecks throughout thetransmission path. Here is a sample output of the TRACEROUTE command:
  3. 3. The Telnet CommandUse the telnet command to verify TCP stack and application layer software betweensource and destination stations. Of course, to be able to telnet on a Cisco device, thelatter needs to be already configured to accept telnet connections. To use TELNETjust issue the telnet command along with the IP address or hostname of the remotestation:The Show Interfaces CommandThe show interfaces command presents all the available interfaces that can beconfigured on your Cisco device. You can explicitly use this command to show onlydetails on a single interface by issuing the interfaces name after the show interfacescommand. This command is very useful because it can reveal layer 1 and layer 2problems. Moreover, this command provides details regarding the hardware address(MAC), IP address, encapsulation method and statistics concerning erroneousconditions on the specific interface. Examine the output of the show interfacefastethernet 0:
  4. 4. The first line of the show interface command is the most important one. The firstparameter refers to the physical layer, thereforein this case FastEthernet 0 is up andoperating. The second part of the line refers to the Data Link Layer; therefore hereline protocol is up means that we have layer 2 connectivity as well. There are fourpossible 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 CommandThe show ip interface command will provide details regarding layer 3 configurationon the interfaces. Using this command you can see the IP address and mask
  5. 5. configured on a given interface, whether an access list is applied on the interface aswell as basicIP information.The Show IP Route CommandUse the show ip route command to find detailed information regarding the routesconfigured on the router. Keep in mind that the router can only route packets to thenetworks listed in its routing table. It is possible that a router cannot reach a networkyou manually configured therefore, that specific route is removed from its table andthat is why you should use the show running configuration command in conjunctionwith show ip route to spot possible inconsistencies.
  6. 6. Show Running-Config and Show Startup-Config CommandsIssue the show running configuration command to find out the whole configurationyour 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 startupconfiguration command to see the configuration commands that will be loaded toyour device the next time it reboots and compare it with your running configurationin order to identify possible unconformities.In the End … Follow Your InstinctsIsolating a network malfunction is not an easy task. In many cases you need to swim
  7. 7. into really deep waters in order to identify the error. Meaning, that you will probablyneed 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; havepatience and follow your gut.Sometimes the problem that you’re searching for will be right in front of you and willslip out of your hands, just when you think you have it. So be persistent, be patientand remember: you will win the battle!Be prepared; in order to fight well, you should be armed well. So get to know yourdevice very well, know how it operates, and know how to use the basictroubleshooting commands – that you just learned – so that you can identify theproblem, or at least estimate where the problem might be. Good luck!More Networking Info and Tips:CCNP TSHOOT: Cisco Troubleshooting Techniques & ProceduresHow to Troubleshoot OSPF?How to Configure EIGRP on a Cisco Router?