VLANs: Why You Need Them and How They Work (Part 3)
VLANs: Why You Need Them and How They Work (Part 1)
VLANs: Why You Need Them and How They Work (Part 2)
What is a Management VLAN, and Should I Use It?
Can I Get Myself Into Trouble Using VLANs?
- Device is on the wrong VLAN: This happens when traffic is sent to the wrong VLAN as it enters the network. Fortunately, this is fairly easy to catch, especially if your client device is configured for DHCP. One look at the IP address on the client device will indicate whether it has a DHCP address on the correct subnet. For static clients, an arping or nmap on the wrong VLAN will reveal the presence of the client. To get your device back on the correct VLAN, make sure your SSID settings and PVID/untagged VLAN switch settings are correct.
- Data traffic doesn’t flow: This results when traffic is sent to the wrong VLAN as it enters the network, or when switch ports are not properly and explicitly configured to pass traffic on that VLAN. Remember that all ports on a switch** should be trunk ports, configured for all tagged VLANs used in the network, including management VLANs. To prevent this issue, remember to configure ports connected to client devices or network appliances for the correct PVID/untagged VLAN for the client.
- Device loses access to network configuration: This is usually the result of a mismatch between the PC used to configure the network devices and the management VLAN set up on the device. Management VLANs should generally be configured last (after devices), because once you set a network device to use a VLAN, you will lose access to the device until its PC port connects to the same VLAN.*** To ensure connection, make sure the PC port used by the device is configured to the management VLAN used by the device.
Editor’s Note: This post was originally published in June 2015 and has been updated.
*Command Line Interface
**Defined here as ports connected to either the router, backhaul to other switches, or access points.
***Example: If you have a switch configured to management VLAN 4000, but none of the switch ports are configured for tagged or untagged access on VLAN 4000, you are cut off from the switch and have no way to access the configuration, short of a serial interface or a hard reset.