Understanding VLANs: Concepts and Configuration Tips

July 2001 · 5 min read

Virtual LANs (VLANs) allow for logical segmentation of Layer 2 networks without requiring physical separation. This provides flexibility, improves security, and enhances network performance.

What is a VLAN?

A VLAN is a broadcast domain created by switches. Instead of grouping devices based on physical location, VLANs enable you to group them logically based on function, department, or application. This decouples network segmentation from physical cabling.

Benefits of VLANs

• Improved network performance through reduced broadcast domains
• Enhanced security by isolating traffic between departments
• Flexibility to move devices without changing physical topology

Basic VLAN Configuration

Here’s a quick look at how to create and assign VLANs on a Cisco Catalyst switch:

Switch# configure terminal
Switch(config)# vlan 10
Switch(config-vlan)# name HR
Switch(config)# interface fa0/1
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 10
  

Remember to assign the correct VLAN to the appropriate interfaces and to use trunking if you want to allow multiple VLANs to traverse a single uplink.

Common Pitfalls

• Forgetting to add the VLAN to the trunk port
• Assigning interfaces to VLANs not configured on the switch
• Using native VLAN mismatches on trunk links

Eduardo Wnorowski is a network infrastructure consultant.
He writes about foundational networking concepts, especially those relevant to campus and enterprise switching.