How to configure and verify VLANs

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


The existence of the VLANs within local area network brings about the division and proper management of the network and improvement in the security system due to the ability to apply only one host to one matching switch. When it comes to the sharing of the broadcasting domain, every network flows in a specific path preventing them from colliding with each other or flooding of the entire network path. VLANs are designed in such a way that they fit each and every environment and hence proven to be user friendly and the fact that they can be used to cover wide area networks without bringing about any complications. The stages of identification of VLANs are quite simple; first of all one identifies the VLANs subnet then the VLANs identifiers that are to be attached to the available switch ports and the last thing is the host that it is deemed to serve.

Configuration of VLANs

Configuration mainly occurs in VLANs so as the user can be able to directly indicate to an IP address or any available subnet and as a result a network link is able to carry multiple VLANs that involves the third layer of the network. These multiple VLANs are usually identified by labels and other available tags usually inserted in the packets. The first step towards configuration of the VLANs assign appropriate users to the available VLANs then the VLAN network is enabled after that, one is able to create the VLAN mainly on the catalyst switch.

The most important thing to do before completing configuration is the use of switch port and it is of benefit since it identifies the fastest Ethernet interface to be used during the process of configuration of the VLANs. In addition this switch port mode can be configured using the different options:

  • Trunk which is a network link carrying a number of VLANs, its main function is to relate with the other partners so as to prepare them before fully participating in the configuration process and are usually identified as labels that are inserted in the packets of the VLANs. For efficiency reasons the trunk is usually organized in a certain design either switch to router or switch to switch
  • Dynamic auto. In this case the interface is usually converted into a trunk once the interface is connected to the correct port and then set to a state of desirability.
  • Dynamic desirable whereby there is a persistent to convert link into a specific trunk and this becomes a success only when the adjacent interface is set one of these states desirable, state of automatic mode or a desirable state. As a result is considered to be the default mode when it comes to interfaces of the Ethernet.
  • Access. This is the last option of switch port mode where its main duty is to disable the port trunk mode. The absence of the trunk system is an assurance that there are no negotiations that take places and as a result the operation is run as usual.

The other option of configuring VLANs is the use of the command that is usually displayed in the show interface of the switch port. This process is quite easy since one is automatically redirected to the VLAN configuration as well as other interfaces that directly relate to it which is the operational mode. In addition to this there are restrictions to be followed before configuring the VLANs.

Restrictions of VLAN configuration

One should not try to configure the VLANs with ports that are not from the second layer since they do not match, one is restricted from removing the VLAN once the translation is in the process of functioning and one is restricted from mixing different kinds of VLANs in the trunk system an example is from native to the present VLAN. There are also certain parameters required before any configuration can be successful some of which are considered to be default like the Ethernet VLAN 1 other parameters that do not use the default mode are VLANs 2 which is configured through route 1001 and many more examples, while in private VLANs the ports are usually configured after a feature known as the VLAN locking is enabled, whereas in cases of default this feature is usually automatically disabled.

Verification of the VLANs configuration

This is an easy stage and does not require one to do a lot of work. The main thing is to select the appropriate command for that matter the show vlan command which in return displays all the available switch ports and the correct VLANs that are linked to them, their status and the necessary parameters that were used to configure the VLANs and also the trunks that are connected to the VLANs. When this is all done and one is satisfied with the available information then verification is considered to be complete.

When verification is done, there are three more stages that are involved in trouble shooting the VLAN configuration. The first thing is to ensure that the physical connectivity is intact this addresses issues concerning cable and the switch port. The next stage is to check the configuration of the interface and make sure that there are no errors or collisions caused by over flow of the networks. The last stage is whether there is communication between VLANs. Lack of communication between them is brought about if the two hosts are from the same VLAN, to be able to solve this issue one is recommended to have a router that will enable communication between the two hosts without facing any difficulties. If the host is not able to connect to the switch after this then one has to ensure that the switches of the VLAN belong to the same subnet.


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