A trunk port (VLAN trunk) on a network switch is capable of handling traffic to and from multiple VLANs. Operating at Layer 2, the trunk port tags a data packet in the frame header designating which VLAN the packet should be delivered to.

The tags are known as 802.1Q tags or Inter-Switch Link (ISL) tags. While the tags allow for more efficient transmission of data, the trunk port can also still deliver untagged packets.