Supporting Multiple VLANs on a Single Trunk
What is a VLAN trunking protocol?

A special protocol is required to establish a trunk link between two devices. A trunk link may exist between these devices:

  • Two switches
  • A switch and a router
  • A Switch and a trunk capable NIC in a node such as a server

If a single physical link carries traffic for multiple VLANs, each frame must be "marked" with a VLAN ID so it is differentiated from frames from other VLANs. This "marking" or frame identification is accomplished through the implementation of a trunking protocol. Frame identification uniquely assigns an ID, referred to as a VLAN ID (VID), to each frame. Each receiving switch examines this VID to determine the destination VLAN of the frame.

VLAN IDs are only associated with frames traversing a trunk link. When a frame enters or exits the switch on an access link, no VLAN ID is present. The ASIC on the switch port assigns the VLAN ID to a frame as it is placed on a trunk link and also strips off the VLAN ID if the frame exits an access switch port.

Trunk links should be managed so that they carry only traffic for intended VLANs. This practice keeps unwanted VLAN data traffic from traversing links unnecessarily. Trunk links are used between the Access and Distribution layers of the Campus Switch Block. These are the trunk protocols used to carry multiple VLANs over a single link:

  • ISL – Cisco Inter Switch Link
  • 802.1Q – IEEE standard trunking protocol