12.3. Tag and Frame Formats
In Chapter 11 we discussed the purpose and use of VLAN tags. In this section we examine the details of the 802.1Q tag format and how it is encapsulated in the various LAN technologies of interest.
An 802.1Q VLAN tag comprises three elements:
The VLAN Protocol Identifier (VPID) field identifies the frame as being VLAN tagged; that is, it is used to differentiate tagged frames from untagged frames.
A Tag Control Information (TCI) field comprises the essential ingredients of the VLAN tag.
An optional Embedded Routing Information Field (E-RIF) allows Source Routing information to be embedded within the VLAN tag for transport across LANs that do not support native Source Routing.
Each of these fields is described in detail in the following text. Following those explanations, we will show how these three fields are inserted into Ethernet, Token Ring, and FDDI frames.
12.3.1. VLAN Protocol Identifier
The VLAN Protocol Identifier (VPID) is a 2-byte field with a value of 0x8100. When present, it indicates that the frame is VLAN tagged and that the next 2 bytes in the frame contain the Tag Control Information. Depending on the type of LAN, the VPID field is encoded in one of two ways:
Ethernet LANs: On an Ethernet, the VPID is used as a protocol type; that is, when the Ethernet Type field (following the Source Address) contains the VPID value, the frame carries a VLAN tag and the next 2 bytes contain the Tag Control Information, as depicted in Figure 12-1.