4.5 Trunking
4.5.1 Trunking overview
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, typically two switches. In the top figure, we can see two VLANs that we want 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 we wanted to add 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, as shown in the bottom figure.

Here, the unique 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 for Ethernet segments follow:

  • ISL (this is the original Cisco proprietary InterSwitch Link protocol)
  • 802.1Q (the IEEE standard we focus on in this section)