Frame tagging

In order to shield or, if necessary, to priorities data traffic of a virtual LAN against the other network users, data packets must have an additional feature (a “tag”). That’s why the respective process is also called “frame tagging”.

Frame tagging must be realized such that the following requirements are fulfilled:

• Data packets with and without frame tagging must be able to exist in parallel on a physical LAN.
• Stations and switches in a LAN, which do not support VLAN technology, must ignore the data packets with frame tagging and/or treat them as “normal” data packets.

The tagging is realized by an additional field within the MAC frame. This field contains two important information for the virtual LAN:

• VLAN ID: A unique number describes the virtual LAN. This ID defines the belonging of data packets a logical (virtual) LAN. With this 12 bit value it is possible to define up to 4094 different VLANs (VLAN IDs “0” and “4095” are reserved resp. inadmissible).
  Note: VLAN ID “1” is used by many devices as the Default VLAN ID. Concerning unconfigured devices, all ports belong to this Default VLAN. However, this assignment can also be changed by configuration.
• Priority: The priority of a VLAN-tagged data packet is indicated by a 3 bit value. “0” represents the lowest priority, “7” the highest one. Data packets without VLAN tag are treated with priority “0”.

This additional field makes the MAC frames longer than actually allowed. These “overlong” packets can only be recognized and evaluated by VLAN-capable stations and switches. Frame tagging incidentally leads to the desired behavior for network users without VLAN support:

• Switches without VLAN support simply pass on these data packets and ignore the additional fields within the MAC frame.
• Stations without VLAN support are not able to recognize the protocol type due to the inserted VLAN tag and discard the packets silently.