Troubleshooting VLAN Assignments – CompTIA Network+ N10-006 – 4.6

A VLAN configuration issue is often difficult to troubleshoot because the issue is caused by a problem on the network switch. In this video, you’ll learn about VLAN issues and how to troubleshoot VLAN problems on your switch.
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VLAN configuration issues can be challenging to troubleshoot. You can’t see the VLAN on your local device. All of the VLAN configurations are done on the switch. Every interface on the switch is assigned to a VLAN, and if you’re connecting to an interface that is assigned to the wrong VLAN, you may run into problems.

I’ve had this happen to me a number of times where I’m putting in a brand new piece of equipment. This device is an infrastructure device, so it’s commonly given a static IP address. So I’m assigned an IP address. I provide the subnet mask for default gateway. I connect to the network and I get a link light, but I’m not able to ping. That’s because I’ve connected to an interface which was assigned to me. That interface was misconfigured with the wrong VLAN, and therefore my IP address and default gateway information are completely incorrect for that VLAN.

If you’re using DHCP, you may be able to get around these problems because you’ll be assigned the correct IP address and subnet mask and default gateway for the VLAN you happen to be connected to. It’s up to the network administrator to make sure that where you’re connecting is the appropriate VLAN for your use, however.

One good thing to know is that if you have been assigned an IP address and you’ve configured it and you’ve connect it to the switch and everything is working, then you’re on the correct VLAN. If you were assigned the incorrect VLAN and you tried to connect with anything given that static IP address and default gateway, you wouldn’t be communicating to anything on the network.

When you’re troubleshooting these VLAN problems, one of the first place is you should goes to your documentation. You do have documentation, don’t you? You should know exactly what VLANs are associated with what interfaces and what those are correlated with. This is the VLAN configuration for a very simple switch that’s in my lab. I have a VLAN 1 and a VLAN 20 on this switch. You can see all of the different gigabit ethernet ports that are configured for VLAN 1. And you can see the small number of interfaces that are configured for VLAN 20.

You can then check to see what interface on the switch you’re plugged into what VLAN has been assigned to that interface, and then compare it to the IP address that you were originally given. Ideally that VLAN and that IP address should both be referring to the same network, and if they’re not, you have a VLAN misconfiguration.

If you’re troubleshooting a VLAN configuration between switches, then you may be configuring one on a trunk link. For this trunk you need to be sure that the VLAN that needs to traverse the link has been assigned as part of the trunk. And you need to look at the interfaces on both sides, which means you’re looking on both switches and confirming that those VLAN configurations are exactly the same.

If you do find there is a VLAN misconfiguration, you may be able to simply change the VLAN to the correct VLAN ID, or you may have to change the VLAN ID and assign a completely different IP address range for the device on the other end of that connection.