Frame Tagging

Frame Tagging
The switch in an internetwork needs a way to keep track of users and frames as they travel the
switch fabric and VLANs. Frame identification, called frame tagging, uniquely assigns a userdefined
ID to each frame. This is sometimes referred to as a VLAN ID or color.
Frame tagging is used to identify the VLAN that the packet belongs to. The tag is placed on
the frame as it enters the first switch it runs into. As long as the frame does not exit out a nontrunk
port, the frame keeps the identifying tag. This enables each switch to see what VLAN the
frame belongs to, and each switch that the frame reaches must identify the VLAN ID and then
determine what to do with the frame based on the filter table. If the frame reaches a switch that
has another trunk link, the frame can be forwarded out the trunk-link port. After the frame
reaches an exit to an access link, the switch removes the VLAN identifier. The end device
receives the frames without having to understand the VLAN identification.
If you are using NetFlow switching hardware on your Cisco switches, this enables devices on
different VLANs to communicate after taking just the first packet through the router. This
means that communication can occur from port to port on a switch, instead of from port to
router to port, when traversing VLANs.