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| 2.5 |  | VLAN Trunking and Dynamic Trunking Protocol (DTP) |
| 2.5.1 |  | Trunking Overview |

When a port is in the trunk mode, the trunk link can be configured to transport all VLANs or a restricted set of VLANs across an entire network. Trunk links are typically used to connect switches to other switches, routers, or VLAN-enabled servers.

In basic terminology, a trunk is a point-to-point link that supports several VLANs. The purpose of a trunk is to save ports when creating a link between two devices implementing VLANs. In the top figure, there are two VLANs that need to be available on two switches, Sa and Sb. The first easy method of implementation is to create two physical links between the devices, each one carrying the traffic for a separate VLAN.

Of course, this first solution does not scale very well. If the network has a third VLAN, we would need to sacrifice two additional ports. This design is also inefficient in terms of load sharing; the traffic on some VLANs may not justify a dedicated link. A trunk will bundle virtual links over one physical link.

In the bottom figure, one physical link between the two switches is able to carry traffic for any VLAN. In order to achieve this, each frame sent on the link is tagged by Sa so that Sb knows which VLAN it belongs to. Different tagging schemes exist. The most common protocols for Ethernet segments follow are the IEEE 802.1Q standard and the Cisco proprietary Inter-Switch Link (ISL) protocol.