Troubleshooting common issues in VLAN

Virtual local area networks (VLANs) are the virtual counterparts of local area networks (LANs). Their characteristics and the devices they are comprised of are the same, but the difference lies in where the two types of networks exist. A LAN exists in the physical environment, while a VLAN exists virtually. VLANs are useful when you want to partition the network at the data link layer by grouping the devices as subsets for isolating traffic. While VLANs make your network management and monitoring hassle-free, they are prone to some problems. Below, we will list some common problems seen with VLANs and explain how they can be resolved.

Native VLAN mismatch

A native VLAN mismatch happens when the interfaces of a switch are configured with different native VLANs. A native VLAN mismatch is also dependent upon the trunking model at both ends. With a native VLAN mismatch, the traffic generated by the PC will be misdirected to multiple VLANs, causing issues in the network. To resolve the native VLAN mismatch, configure the same VLAN on all the interfaces that are connected to the hub so that the traffic generated will only be redirected to one VLAN.

Duplex mismatch

When it comes to your attention that there are many collisions on the interface and that the data transfer rate has slowed down, you should inspect the issue of a duplex mismatch. A duplex is a point-to-point communication system between two devices. In a duplex, two devices can communicate with each other in either both directions or just one direction. Such collisions are also caused by congestion. High traffic flowing from one point of the network would naturally slow down the speed of data transfers.

Duplex mismatches can also be caused by the following:

  • Slow collision domain connectivity: Slow collision domain connectivity can cause a duplex mismatch. A collision domain is comprised of connected devices with half-duplex configuration that are all connected to each other or a hub. During a duplex mismatch, problems occur specifically in one collision domain, while other domains in the same VLAN perform normally.
  • Traffic loops: Traffic loops are one of the most common causes of a slow VLAN. The traffic from one switch to another gets looped, thereby overloading the traffic. The looped packets overflow on network interface cards (NICs), slowing down performance in the VLAN and causing packet loss.

The solution to a duplex mismatch is to set the interface to the correct bandwidth and check that the data is being routed properly and that there are no issues in the process of routing.

A lack of communication between a host and a switch

A faulty connection between a host and a switch can make the process of communication in a VLAN erratic. In some cases, the host may even be unable to connect with the switch. Such problems persist because the host is not in the same subnet as the switch in the VLAN. Therefore, you should always make sure that both the host and the switch are in the same subnet.

Problems with physical connections

VLANs are comprised of mostly virtual components and connections, but there are still a few physical connections that needs to be maintained. Whenever there is a problem, it is best to first check if the LED lights on physical switches are blinking. This signifies that the physical layer, or Layer 1, is working properly. Ensure that the cables are properly connected and in good condition, without signs of wear and tear. Faulty physical components can result in data flowing in the wrong direction on the interface of the switch. The issue could be a faulty cable or NIC. Thus, you need to change the cable or NIC and check whether the problem still persists.

Common issues in inter-VLAN routing

Missing VLANs

Every VLAN has an assigned port, enabling it to be a part of the network. However, there have been instances where a port on a switch goes inactive. This instance of an inactive port occurs because the VLAN assigned to the port has been deleted for some reason. To resolve this, you can use the "show VLAN" command to search through the table of available LANs and check if any of them has been deleted. If you find that one has been deleted, you will have to create it and add it again.

Switch trunk port issues

Misconfigured switch ports are a common issue found in inter-VLAN routing. They result from a connecting router port not being assigned to the correct VLAN. To resolve this issue, first ensure that the trunks are configured correctly using the "show interfaces trunk" command. Then, verify that the port is a trunk port and that it is enabled using the "show running-config" command.

Switch access port issues

A port configured as an access port may not be configured to be in a particular VLAN. Such an issue can be resolved by first verifying that the port is indeed an access port and that it is enabled. Assign the correct VLAN to the access port. Also make sure that the host is configured correctly in the right subnet.

VLANs tick two boxes in network management: better network performance and better security. You can carry out network management efficiently while also conserving network resources by preventing the flooding of traffic within the network. A basic understanding of the issues mentioned above and how to configure and troubleshoot can help you quickly isolate problems in your network and work towards long-term resolutions.

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