Trunk Port

Definition

A Trunk Port is a switch port that carries traffic for multiple VLANs over a single physical cable. Instead of connecting end devices, it usually connects switch-to-switch, switch-to-router, or switch-to-server links. In the VLAN process, it transports VLAN traffic between networking devices.

What Problem It Solves

Without Trunk Ports, every VLAN would require its own physical cable between switches. As the number of VLANs grows, cabling becomes expensive and difficult to manage.

What Happens If Not Used

VLAN 10, VLAN 20, and VLAN 30 could not efficiently reach another switch. You would need separate links for each VLAN.

Easy Wording

A Trunk Port is one cable that safely carries traffic for many VLANs at the same time.

Layman Example

Imagine a highway with lanes for buses, cars, and trucks. All vehicles use the same road, but lane markings keep them separated. VLAN tags are like those lane markings.

Technical Example

• PC-A sends data through an Access Port (VLAN 10).
• The switch identifies the frame as VLAN 10.
• Before sending it over the Trunk Port, the switch adds an 802.1Q VLAN tag.
• The receiving switch reads the tag and knows the frame belongs to VLAN 10.
• It removes the tag and forwards the frame through the correct Access Port.

Limitation: Trunk Ports rely on VLAN tags to identify traffic. The next concept explains 802.1Q VLAN Tagging, which makes this possible.