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    Networking at Layer 2: Forwarding VLAN Tagged Frames

    VLAN tags were not developed as a way to limit network node table entries. They were originally invented to allow LAN switches to distinguish between physical groups of LAN ports and logical groups of LAN ports. In other words, there was a need to configure a LAN switch (or group of local LAN switches) to know that “these ports belong to VLAN A” and “these ports belong to VLAN B.”

    This was important because of how all LANs, not just Ethernet, work at the frame level. Lots of frames on a LAN are broadcast to all stations (hosts and network nodes) on the LAN segment. Also, multicasting works by flooding traffic within the VLAN. The stations that received broadcast frames form the broadcast domain of the LAN. Only Ethernet frames belonging to same broadcast domain are forwarded out certain ports on the LAN switch. This prevents broadcast storms and isolates routine control frames onto the LAN segment where they make the most sense.

    The VLAN tag was invented to distinguish among different VLAN broadcast domains on a group of LAN switches. The VLAN tag is a two-byte field inserted between the source MAC address and the Ethertype (or length) field in an Ethernet frame. Another two-byte field, the Tag Protocol Identifier (TPI or TPID), precedes the VLAN tag field.

    Two fields were necessary to hold one piece of information, the VLAN tag, to enable receivers to distinguish between untagged or plain Ethernet frames and those containing VLAN tags. A mechanism was required to differentiate between the Ethertype and length field for the untagged case and to distinguish among VLAN tag, Ethertype, and length field for the tagged case. The answer was to constrain the TPID field to values that were not valid Ethernet frame lengths or defined as valid Ethertypes. The first VLAN tag added to an Ethernet frame is always indicated by a TPID value of 0x8100. This is not the VLAN identifier, which appears in the next two bytes.

    In Figure 1, a native or normal Ethernet frame is compared to a VLAN-tagged Ethernet frame. The lengths of each field, in bytes, is shown next to the field name.

    Figure 1: Native (Normal) and VLAN-Tagged Ethernet Frames

    Native (Normal) and VLAN-Tagged Ethernet
Frames

    The VLAN tag subtracts four bytes from the total MTU length of the Ethernet frame, but this is seldom a problem if kept in mind. When this tag is used in an Ethernet frame, the frame complies with the IEEE 802.1Q (formerly IEEE 802.1q) specification.

    Together, the four added bytes form the VLAN tag, but the individual fields that comprise it are more important. The 2–byte TPID field is just a number and has no structure, only having allowed and disallowed values. However, the 2-byte Tag Control Information (TCI) field has a defined structure:

    • The three bits of the User Priority field are defined by the IEEE 802.1p specification. These can mimic class-of-service (CoS) parameters established at other layers of the network (IP precedence bits, or MPLS EXP bits, and so on).
    • The Canonical Format Indicator (CFI) bit indicates whether the following 12 bits of VLAN identifier conform to Ethernet or not. For Ethernet frames, this bit is always set to 0. (The other possible value, CFI=1, is used for Token Ring LANs, and tagged frames should never be bridged between an Ethernet and Token Ring LAN regardless of the VLAN tag or MAC address.)
    • The 12-bit VLAN ID allows for 4096 possible VLANs, but not all values are used in all cases.

    Modified: 2017-08-31