Trunks are used to interconnect switches to form networks, and to interconnect local area networks (LANs) to form wide area networks (WANs) or virtual LANs (VLANs). A trunk often consists of multiple wires, cables, or fiber optic strands to maximize the available bandwidth and the number of channels that can be accommodated. A trunk can also be abroadband wireless link. The use and management of trunks in a communications system is known as trunking. It minimizes the number of physical signal paths, and thus the total amount of cable hardware, required to serve a given number of subscribers in a network.

In Cisco networks, trunking is a special function that can be assigned to aport, making that port capable of carrying traffic for any or all of the VLANs accessible by a particular switch. Such 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. An access port does not provide such tags, because the VLAN for it is pre-assigned, and identifying markers are therefore unnecessary.