Troubleshoot and resolve trunking problems on Cisco switches

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


Troubleshooting is basically the method that is followed to solve problems that may arise in a system or in a process. We in this chapter will specifically deal with how to troubleshoot and resolve trunking problems on Cisco switches. As a network administrator you may have to deal with a situation more frequently. Apart from the theory that we are about to discuss it will be advisable that you try some practical assignments too in order to understand the process better.

We will try to cover most of the things that you need to know about this topic in this chapter itself. First of all let’s try to understand what trunking is all about. Well trunking is a method that is followed to transmit data across a system. The system is compared to as a tree that has many branches. That is where the name has come from. This is mostly used in VHF (very high frequency) equipments and other telecommunication systems. These are devices that work on many signals at the same time as a result trunking is a necessity. This process increases the speed of the telecom devices and increases the bandwith. The data that is transmitted using the trunking process can be audio, vedio or even images.

Trunking is often used in Cisco switches too we will now discuss in this chapter the process in more details. The trunk links are generally used to pass the VLAN information across the switches. The port that is used on a Cisco switch is either a trunk port or an access port. The trunk port is a default member that is common to all VLAN as a result they carry traffic for all the VLANs. This traffic flow is between the switches. Sometime there can be problem that arises in this traffic transmission. The reason for these problems can be many. As a network administrator you will be required to solve these problems and that is what we are about to discuss in this chapter.

Some points that you must remember before you try to troubleshoot the problems are as follows:

  1. Each of the VLAN must to an IP subnet that is unique.
  2. Two devices in the same VLAN have different subnet addresses then they will not be able to communicate.
  3. If the above two is the problem then the solution is easy. All that you need to do is identify the configuration that is incorrect and change the subnet address with the correct one.
  4. Incorrect IP configuration is the most common error that you will often find in a VLAN. This is exactly why you must learn to troubleshoot this problem with perfection.
  5. You can use the show VLAN command to check if the port belongs to the correct VLAN. If you find that he VLAN assigned is a wrong one then you can use the switchport access VLAN command to correct the VLAN membership. Using the show mac address-table command you can check which address was assigned to which port of the switch. If the VLAN to which the port is assigned is deleted the port will automatically become inactive. This is a point that you must keep in mind. This ensures that the VLAN is unable to communicate with the remaining network.
  6. You can use the show interface f0/1 switchport command to find out if a port is actually inactive. The port will be inactive unless the missing VLAN is created again. You can create the VLAN using the command vlan vlan _id.
  7. As a network administrator you will often find links that are incorrectly acting as trunk links. It does happen that a switch port starts to behave like a trunk port. This process is also called VLAN leaking in technical terms. This can also happen due to a truck that was not configured correctly. To solve this problem you can use the enclosed method:
    • Using the show interface trunk you will be able to check peers that match the VLAN. If the native and the VLAN is not matching in both the ends then the VLAN leaking will occur for sure.
    • Now you must use the show interface trunk command to check if a trunk was created between the switches. You must try to configure the trunk links whenever there is a possibility.
  8. In order to check the status of the trunk you must try to determine the native VLAN. Using the show interface trunk command you can also verify the trunk establishment. If there is a native VLAN mismatch there will be a security risk and the result will not be one that we intend to see.
  9. You will often see that there will be a connectivity issue if there was a VLAN mismatch. The data that is associated with either of the native VLAN will not support the trunk link must be kept in mind.
  10. If there is a trunk mode mismatch there will be a loss in the connectivity of the network.
  11. If the VLANs are allowed on trunks then an unexpected traffic will be created. As a result no traffic will be sent over the trunk.
  12. Considering the above points as you troubleshoot for a trunking problem you must start with native VLAN mismatch. If this is not the cause you can check the trunk mode mismatch and finally as the last step check the VLAN list that is allowed on the trunk.
  13. As a second step to troubleshoot check the status of the trunk ports on the different switches. Most of the VLAN problems occur as the trunk port is not compatible with the neighbouring trunk ports.
  14. First the traffic must be allowed on the trunk to do so the command that you will be using is switchport trunk allowed vlan vlan – id command.

The topic of troubleshoot and resolve trunking problems on Cisco switches is very important from the exam point of view so try to spend a good amount of time preparing this section from CCNA exam perspective. Just try to prepared based on these points that we have mentioned and you will definitely be able to do well in this section.


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