Troubleshoot and resolve VLAN problems

Exam: Cisco 200-120 - Interconnecting Cisco Networking Devices: Accelerated (CCNAX)


As you prepare for the CCNA exam one subtopic under the section troubleshooting that you must study well is that on “troubleshoot and resolve VLAN problems”. We in this chapter will discuss this section in details. VLAN has the ability to isolate devices that are physically connected to a network but if you look at these devices they are logically part of different LANs. The LANs that are not connected to each other may not be aware of each other’s existence but this cannot work all the time. This is when the VLAN problems occur. As a network administrator you must know well how exactly to manage a situation like this. It is a common scenario that you will come across very often. How to deal with these problems is what we will be discussing now.

Before you solve the problems in VLAN you must take care of two things. These are that you must have all the information that you need about the connectivity and the configuration of the individual devices that are connected to the network. Some of the checklist that you must keep in mind about the VLAN trouble shoot is as follows:

  1. Check that the physical connectivity of the network is fine for any problems in the port
  2. One must also check that the both the end of the device is in the same VLAN

To get the VLAN information some of these commands can be used:

  • Show VLAN internal errors
  • Show VLAN all-ports
  • Show VLAN id vlan-id

Identify that VLANs are configured

Each of the VLAN will have a separate broadcast domain. This allows the ports that belong to a certain VLAN to share broadcasts. This also eliminates unnecessary traffic. As a result the performance of the network improves. VLANs can also be used across WAN (wide area network). This can be done using a layer 2 tunnel. It is quite easy to configure a VLAN. During a VLAN configuration the following problems may troubleshoot:

  1. If the two hosts are not able to communicate during the configuration just check if they are in the same VLAN. If the VLANs are different then you will need a router to ensure that these are connected. If there is a problem connecting to the switch then you must ensure that the switch and the host belong to the same subnet as the VLAN.
  2. Checking the physical connectivity is the first step as you deal with the troubleshooting of a VLAN. The cable and the switch port must be in a good condition always.
  3. The interface configuration of the switch must be checked before you try to get deeper into the problem. The command show interface (interface number) can be used to locate the CRC errors. Most of these problems are a result of bad cable and NIC.

The hosts in the network must always be differentiated. This can be done using departmental differentiation. By doing this you will have a group that needs isolation for a better performance and a better security. With the help of VLAN one can ensure that the network has better security and performance.

Verify port membership correct

As you configure the VLAN you must give it an ID number. The port channels the many physical interfaces to one logical interface. The trunking on the other hand allows the interconnected to transmit frames among the VLAN. This basically allows a trunk to be carried in multiple VLAN traffic. The enclosed checklist can be used in there is a problem in the port channel:

  1. The CLI command will list the port channel requirements.
  2. Both the sides of the port channel must be connected to the same number of interfaces.
  3. Each interface must be connected to the same type of the interface.
  4. All the VLANS in a trunk port must be allowed in the VLAN list.
  5. The port channel configuration must be present in the profile that is being used by the physical ports
  6. You must configure the APC if the ports are connected to different switches.
  7. All the members who are trying to form a port channel must be on the same module.
  8. All the interfaces in the port channel must have the same destination device.

Some of the commands that are used in troubleshooting of the port channel are:

  1. Show port channel summary
  2. show port-channel internal event-history interface port-channel (channel number)
  3. show system internal ethpm event-history interface port-channel(channel number)

Correct IP address is configured

The IP address must be assigned correctly to the network interface. When the correct IP address is configured this will allow the interface to communicate with the other hosts in the interface.

The IP address basically gives a location to which the IP datagrams can be sent. The IP addresses are kept for some special uses and cannot be used for the host or for the subnet. The official IP address can be found in the RFC 1166. Every interface can have only one primary IP address.

Some of the errors that can be found while troubleshooting the correct IP address:

  1. Check the configuration of the IP address. If the configuration is blank then you will have to assign an IP address. Using the command IPCONFIG /ALL you can see the TCP/IP configuration. This can happen if the network card was not installed properly. Some of the network cards can be bad and some can be good.
  2. The second error can be if there is a communication failure. You must also see if the IP address was manually entered correct at the first place. You can also ping the IP address of the computer if it was successful then it means that the TCP/IP stack is functioning.

We hope that this chapter on the topic “Troubleshoot and resolve VLAN problems” will help you get a clearer idea on this topic. Try to spend a good amount of time on this topic in order to get a better score in the CCNA exam.


Related IT Guides

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  3. Describe the operation and necessity of using private and public IP addresses for IPv4 addressing
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  5. Identify basic switching concepts and the operation of Cisco switches
  6. Recognize High availability (FHRP)
  7. Shutdown unused ports
  8. Troubleshoot and resolve interVLAN routing problems
  9. Troubleshoot and resolve routing issues
  10. Troubleshoot and resolve spanning tree operation issues
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