How to Troubleshoot VLAN Problems

Certification: Cisco CCNA Routing and Switching - Cisco Certified Network Associate (CCNA) Routing and Switching


VLAN is also known as “Virtual Local Area Network”. The basic purpose of a VLAN is to make sure that there is no physical restriction on the connection of layer 2 switches. These networks hold the ability to span different switches without having any physical restriction. It also adds an extra layer of security by separating the physical networks from the logical ones.It is extremely essential that all the components of a VLAN, both physical and virtual, work perfectly to ensure the smooth flow of processes in the network. However, there are still a number of problems which are encountered during this process. Following, you will find some of the most common problems and ways to troubleshoot them.

Faulty Physical Connections

We need to keep in mind that although the VLAN makes use of a number of virtual connections and components, there are still certain physical connections which need to be maintained. Firstly, whenever you experience any problem, you need to check that the LED lights on physical switches are blinking. This would ensure that the layer 1, the physical layer, is working properly and is not going through any problem. The next step which you need to do is to check whether the cables are connected properly. You should also make sure that the cables are in a good condition and are not worn or cut from any position.

Another common problem which is observed is the late receiving and sending of data on the interface of the switch. This problem might also have another origin, but it is commonly associated with faulty physical components. The issue due to which this problem occurs could be a faulty cable or NIC. In this regard, you might need to consider changing any of these to check whether the problem or resolved or it still pertains. If you find that the problem is not resolved by doing any of these steps, then you should realize that the problem is more technical and move towards other issues to identify the problem.

Duplex Mismatch or Congestion

If you realize that there are a number of collisions on the interface and the transfer of data has become slow, then you should consider having a look at the issue of Duplex Mismatch.  A Duplex is referred to as a point-to-point communication between two devices which can communicate with each other in either both directions or just one direction. You should make sure that the both of the devices operate on the same duplex so that the connection remains active.

Another reason why these collisions might occur is due to congestion. When a lot of traffic is being transmitted through a single point, then there is going to be a lot of traffic which would restrict the speed of data transfer. Under such conditions, you should make sure that the data is being routed properly and there is no issue in the process of routing.

Lack of communication between two hosts

If you observe that two devices or hosts in the VLAN are not able to communicate, the first question which comes to your mind should be whether they are in the same VLAN or not? You should be aware of the fact that two hosts can only communicate with each other if they are in the same VLAN. However, if they are not, then there is a need for a router to be in between. This router will enable a communication link to be established between the devices in these VLANs. In this regard, you should insert a router in between to enable them to communicate.

This problem of communication can be caused due to one other reason. You should be aware of the fact that one VLAN has only one IP subnet. In this regard, it needs to be ensured that every host or device in a VLAN has the same subnet address. If two hosts on the VLAN have different subnet addresses due to any reason, they will not be able to communicate in any way. Hence, you should check for their subnet addresses and ensure that they have not been changed due to any reason.

Lack of Communication between a host and a switch

If you observe that the process of communication in a VLAN is not taking place properly, then you should consider the possibility of a faulty connection between a host and the switch. There are certain instances where the host is not able to connect with the switch at all. This problem arises because the host is not in the same subnet in which the switch is in the VLAN. Hence, you should make sure that both of them are in the same subnet.

Unable to create a VLAN

When you create a VLAN, you might come across a number of issues without realizing you fault. This is because there are some factors which are tricky enough to be considered. Whenever you fail to create a VLAN, you should have a look at you VLAN ID. You should ensure that you are not using a VLAN ID which has been reserved. This is an issue which many people overlook not considering the consequences. You should keep in mind that the VLAN IDs from 3968 to 4047 as well as 4094 have been reserved for internal use and cannot be used for any other purpose.

Problem of Missing VLANs

Each port is assigned to a particular VLAN to enable it to be a part of that network. However, there have been issues where the port on a switch becomes inactive. This is because the VLAN which was assigned to the port have been deleted due to some reason. If this happens, you should make use of the “show VLAN” command to search through the table of available LANs to see if any of them has been deleted. If it has been deleted, you will have to create and add it again.


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