A trunk is a line or link designed to handle many signal s simultaneously, and that connects major switching centers or node s in a communications system.The transmitted data can be voice (as in the conventional telephone system) data, computer programs, images, video or control signals.uch a port is called a trunk port, in contrast to an access port, which carries traffic only to and from the specific VLAN assigned to it. A trunk port marks frames with special identifying tags (either ISL tags or 802.1Q tags) as they pass between switches, so each frame can be routed to its intended VLAN.