VLANs and Trunking
Common Ethernet switching features
What you will learn
- Explain why VLANs are used to logically segment the broadcast domain.
- Distinguish an access port from a trunk port by purpose.
- Explain the 802.1Q tag's function on a trunk link.
- Recall the native VLAN concept and its relation to untagged frames.
One physical switch can host several VLANs — each behaving as a completely separate network in terms of broadcast domain, with no need for an extra switch or cable. This saves hardware cost and improves security by logically separating traffic.
Core concepts
| VLAN | A separate logical network on the same physical hardware — its devices don't share a broadcast domain with another VLAN except through a router. |
|---|---|
| Access Port | A port assigned to a single VLAN only, usually connected to an end device. |
| Trunk Port | A port carrying multiple VLANs' traffic together, tagged with 802.1Q, used between switches or between a switch and a router. |
| Native VLAN | The one VLAN whose traffic travels untagged over a trunk link — any untagged frame is assumed to belong to it. |
Access vs. Trunk
| Access | Trunk | |
|---|---|---|
| VLANs supported | One only | Several together |
| Tagging | No tagging — the end device doesn't understand 802.1Q | Tagged with 802.1Q for every VLAN except the native one |
| Typical use | A computer, printer, or IP phone | Between two switches, or between a switch and a router |
The native VLAN trap
If the native VLAN differs between the two ends of a trunk link, each end misinterprets the other's traffic — unintended traffic leakage between VLANs, a known security weakness called VLAN hopping through exactly this path.
Quick check
Answer in your head first, then reveal.
What's the key difference between an access port and a trunk port?Reveal the answer
Access carries only one VLAN; trunk carries several tagged VLANs together.
What is the function of the 802.1Q tag?Reveal the answer
Identifying which VLAN each frame belongs to on the shared trunk link.
What security risk results from mismatched native VLANs across a trunk?Reveal the answer
Unintended traffic leakage between VLANs, known as VLAN hopping.
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Your next step
Read the lesson, then mark it complete
Sources
Used to verify the facts. The writing is original to Passuit.