VLAN fundamentals

To enable a network device to identify frames of different VLANs, a VLAN tag field is inserted into the data link layer encapsulation.

The format of VLAN-tagged frames is defined in IEEE 802.1Q issued by the Institute of Electrical and Electronics Engineers (IEEE) in 1999.

The Ethernet II encapsulation format is used here. Besides the Ethernet II encapsulation format, Ethernet also supports other encapsulation formats, including 802.2 LLC, 802.2 SNAP, and 802.3 raw. The VLAN tag fields are added to frames encapsulated in these formats for VLAN identification.

In the header of a traditional Ethernet data frame, the field after the destination MAC address and the source MAC address is the Type field, which indicates the upper layer protocol type, as shown in Figure 39.

IEEE 802.1Q inserts a four-byte VLAN tag after the DA&SA field, as shown in Figure 40.

The fields of a VLAN tag are as follows:

• TPID—The 16-bit TPID field indicates whether a frame is VLAN-tagged. By default, the TPID value is 0x8100, which indicates that the frame is VLAN-tagged. Devices vendors can set the TPID to different values. For compatibility with these devices, modify the TPID value so that frames carry a TPID value identical to the value of a particular vendor, allowing interoperability with devices from that vendor. The device determines whether a received frame carries a VLAN tag by checking the TPID value. When the TPID value of a frame is the configured value or 0x8100, the frame is considered as a VLAN-tagged frame. For information about commands used to modify TPID values, see Layer 2—LAN Switching Command Reference.

• Priority—The 3-bit priority field indicates the 802.1p priority of the frame.

• CFI—The 1-bit CFI field indicates whether the MAC addresses are encapsulated in standard format when packets are transmitted across different media. A value of 0 indicates that MAC addresses are encapsulated in standard format. A value of 1 indicates that MAC addresses are encapsulated in a non-standard format. The value of this field is 0 by default.

• VLAN ID—The 12-bit VLAN ID field identifies the VLAN that the frame belongs to. The VLAN ID range is 0 to 4095. Because 0 and 4095 are reserved, a VLAN ID actually ranges from 1 to 4094.

A network device handles an incoming frame depending on whether the frame is VLAN tagged, and information about the VLAN tags, if any. For more information, see "Introduction to port-based VLAN."

	NOTE: When a frame carrying multiple VLAN tags passes through, the switch processes the frame according to its outer VLAN tag, and transmits the inner tags as payload.