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