In a traditional situation, a network with four VLANs would require four physical connections between the switch and the external router.

As technologies such as Inter-Switch Link (ISL) became more common, network designers began to use trunk links to connect routers to switches. Although any trunking technology such as ISL, 802.1Q, 802.10, or LAN emulation (LANE) can be used, Ethernet-based approaches such as ISL and 802.1Q are most common.

The Cisco Proprietary protocol ISL as well as the IEEE multivendor standard 802.1q are used to trunk VLANs over Fast Ethernet links.

The solid line in the example refers to the single physical link between the Catalyst Switch and the router. This is the physical interface that connects the router to the switch.

As the number of VLANs increases on a network, the physical approach of having one router interface per VLAN quickly becomes unscalable. Networks with many VLANs must use VLAN trunking to assign multiple VLANs to a single router interface.

The dashed lines in the example refer to the multiple logical links running over this physical link using subinterfaces. The router can support many logical interfaces on individual physical links. For example, the Fast Ethernet interface FastEthernet 0/0 might support three virtual interfaces numbered FastEthernet 1/0.1, 1/0.2 and 1/0.3.

The primary advantage of using a trunk link is a reduction in the number of router and switch ports used. Not only can this save money, it can also reduce configuration complexity. Consequently, the trunk-connected router approach can scale to a much larger number of VLANs than a one-link-per-VLAN design.