Networks in small environments are relatively easy to manage because only a limited number of features need to be implemented to achieve the business’s goals. However, in an enterprise environment you will not be using small workgroup switches and SOHO devices but, rather, high-end devices that are capable of optimizing traffic flow by offering a number of advanced functionalities.
One particular feature that might be configured in such an environment is logically separating different network areas using VLANs. Problems can appear when you have configuration issues related to a particular VLAN and this can become difficult to troubleshoot. One way of doing this is to analyze the entire configuration on the switch to identify the problem.
VLAN-related problems are usually detected by observing a lack of connectivity between network machines (e.g., a user cannot ping a server), even though Layer 1 seems to be operating without problems. One important characteristic of VLAN-related problems is they do not generate any performance degradation on the network. If you misconfigure a VLAN, the connection will simply not work, especially considering that VLANs usually separate IP subnets so that only devices within the same VLAN will be able to communicate with each other.
The first step in troubleshooting VLAN problems is to review the documentation and the logical diagrams developed in the design phase so you can identify the span area for each VLAN, including the associated devices and ports on each switch. The next step is to inspect each switch configuration to try to find the problem by comparing them with the documented solution.
You should also verify the IP addressing scheme. If you are statically assigning IP addresses to devices, we may want to go back and check the device to be sure that it has the proper IP and subnet mask combination. If there are any mistakes in the IP addressing scheme, such as configuring devices on the wrong network or with a wrong subnet mask/default gateway, then you will have connectivity problems, even though you have the correct VLAN configured on the switch.
You also want to confirm that the trunk configuration on the switches is correct. If there are multiple switches, they usually have uplinks between them and the VLANs are carried across those uplinks. These inter-switch links are often configured as trunks to allow communication across multiple VLANs. The VLAN has to be a member of the trunk group so that data can be sent from one switch to the other. You also have to make sure that the switch configuration on both sides is set up properly.
If you move a device to another VLAN, you will have to make the change on both the switch and the client itself because it will now have a different IP address in a different IP subnet. If you follow all of these VLAN troubleshooting methods, you can be sure that when you plug in devices for the first time or move them from VLAN to VLAN you will have exactly the connectivity desired.