Understanding How VLANs Operate

802.1Q VLAN operation differs slightly from how a switched networking system operates. These differences are due to the importance of keeping track of each frame and its VLAN association as it passes from switch to switch, or from port to port within a switch.

VLAN-enabled switches act on how frames are classified into a particular VLAN. Sometimes, VLAN classification is based on tags in the headers of data frames. These VLAN tags are added to data frames by the switch as the frames are transmitted out certain ports, and are later used to make forwarding decisions by the switch and other VLAN aware switches. In the absence of a VLAN tag header, the classification of a frame into a particular VLAN depends upon the configuration of the switch port that received the frame.