VLANs & Trunking Made Simple
A VLAN (Virtual LAN) splits one physical switch into multiple logical networks, so you can separate departments, improve security and reduce broadcast traffic. A trunk is a link that carries many VLANs between switches using 802.1Q tags.
Why VLANs exist
Without VLANs, every device on a switch shares one broadcast domain. VLANs let you logically group devices — for example HR, Sales and Servers — even if they share the same switches, improving security, performance and manageability.
Access vs trunk ports
- Access port: assigned to one VLAN, used for PCs, printers and phones.
- Trunk port: carries multiple VLANs between switches (or to a router), tagging each frame so the other side knows which VLAN it belongs to.
802.1Q tagging and the native VLAN
The IEEE 802.1Q standard inserts a 4-byte tag into Ethernet frames on a trunk, identifying the VLAN. The native VLAN is the one exception — its traffic is sent untagged. Mismatched native VLANs are a common real-world error.
Inter-VLAN routing
VLANs are isolated by design, so to let them talk you need Layer 3 routing: either router-on-a-stick (one trunk to a router with a subinterface per VLAN) or a Layer 3 switch using Switched Virtual Interfaces (SVIs).
Try it in Packet Tracer
- Create VLANs:
vlan 10,vlan 20. - Assign access ports:
switchport mode access/switchport access vlan 10. - Configure the uplink as a trunk:
switchport mode trunk. - Add inter-VLAN routing on a router or L3 switch.
Set up your own lab with our home lab guide, or learn it hands-on in our CCNA course.
Frequently asked questions
What is the difference between an access port and a trunk port?
An access port belongs to a single VLAN and connects end devices. A trunk port carries traffic for multiple VLANs between switches and tags frames with 802.1Q.
What is the native VLAN?
The native VLAN carries untagged traffic on a trunk. By default it is VLAN 1; it should match on both ends of the trunk to avoid issues.
How do devices in different VLANs communicate?
Through inter-VLAN routing — either router-on-a-stick (a router subinterface per VLAN) or a Layer 3 switch using SVIs.
Related articles
Want hands-on training?
Learn this on real Cisco lab devices with placement support at Attila Technologies, Ahmedabad.