Explicit VLAN Tagging

Ethernet has been around since the mid-70s, and the maximum length frame (less preamble) was always 1518 bytes. For industrial automation, this frame size is quite large since I/O messages are short. However, after all these years it appears that 1518 bytes are still not enough. The IEEE 802.1Q committee decided that four more bytes were needed in order to define a universally acceptable VLAN tag. There were concerns that stations and hubs could not handle an oversized frame and this new standard required a revision to IEEE 802.3. Everything we said about maximum frame size is now wrong. It is not 1518 bytes, but 1522 when VLAN tags are appended.

The IEEE 802.1Q VLAN tagging scheme is called an explicit VLAN scheme since something (VLAN tag itself) is appended to the frame instead of being implied (implicit VLAN) by the contents of the frame. The four-byte tag is inserted immediately after the source address and before the Type/Length field (see Figure 3). The first two bytes are called the Tag Protocol Identifier which functions much like the Type/Length field. The contents of the two bytes are 0x8100, which is to be recognized as a VLAN tag. The following two bytes are the Tag Control Information. The remainder of the Ethernet frame stays the same except the Frame Check Sequence (FCS) must be recalculated because of the longer frame. Other than that restriction, a VLAN tag can be added or removed without affecting the contents or nature of the message.

Figure 3 — 802.1Q VLAN standard inserts a four-byte tag into a standard Ethernet frame.

The two-byte Tag Control Information consists of three bits for IEEE 802.1p priority levels (that has nothing to do with VLANs), one bit called the Canonical Format Indicator (CFI) and 12 bits for the VLAN identifier. With 12 bits of identifier, there could be up to 4096 VLANs. However, all ones are reserved and all zeros indicate no VLAN association, meaning that the tag is solely to indicate priority level. All other identifiers can be used to indicate a particular VLAN along with the 802.1p priority level of the message.

The CFI bit is used to indicate bit ordering within frames, which is an issue when communicating over non-Ethernet LANs. Since we are only interested in Ethernet LANs, the CFI bit is set to zero.

VLAN-Unaware End Stations and Switches

Since 802.1Q arrived over 20 years after the invention of Ethernet, there are plenty of VLAN-unaware devices in the field. Although an end station will probably accept the elongated frame, will the software driver "choke" on receiving a 0x8100 Ethertype protocol identifier that it has never seen before? The best practice is for end stations never to see VLAN tags unless they are conditioned to do so. With the amount of legacy equipment in the field, it is a good bet the end the end stations are VLAN-unaware. A VLAN-aware end station is one that can receive and apply 802.1Q VLAN tags and, therefore, is termed tag-aware. However, the same is not true of switches. A VLAN-aware switch must be able to make VLAN-port associations but it may not understand 802.1Q tagging. A Port VLAN switch is a good example. A tag-aware switch understands 802.1Q tagging and can make VLAN-port associations as well.