Supporting Multiple VLANs on a Single Trunk
What is a VLAN trunk?

Multiple VLANs are supported between switches through the use of VLAN trunks. A trunk is a Layer 2 link between switches which are running a specialized trunking protocol. Trunks carry the traffic of multiple VLANs over physical links (multiplexing) and enable the extension of a single Layer 2 VLAN between switches.

If frames from a single VLAN traverse a trunk link, a trunking protocol must mark the frame to identify its associated VLAN as the frame is placed onto the trunk link. The receiving switch then knows the frames VLAN of origin and can process the frame accordingly.

On the receiving switch, the VLAN identifier is removed when the frame is forwarded onto an access link associated with its VLAN.