VLAN Tags
Definition and Function
A switch identifies packets from different VLANs according to the information contained in the VLAN tags. IEEE 802.1Q adds a 4-byte VLAN tag between the Source address and Length/Type fields of an Ethernet frame, as shown in Figure 4-2.
A VLAN tag contains four fields. Table 4-1 describes the fields.
| Field | Length | Description | Value |
|---|---|---|---|
| TPID | 2 bytes | Tag Protocol Identifier (TPID) indicates the frame type. | The value 0x8100 indicates an 802.1Q-tagged frame. An 802.1Q-incompatible device discards 802.1Q frames. The value of field is 0x8100 under the IEEE 802.1Q protocol. However, manufacturers can define their own TPID values and users can then modify the value to interconnect with devices from different manufacturers. |
| PRI | 3 bits | Priority (PRI) indicates the frame priority. | The value ranges from 0 to 7. A larger value indicates a higher priority. If congestion occurs, the switch sends packets with higher priorities first. |
| CFI | 1 bit | Canonical Format Indicator (CFI) indicates whether a MAC address is encapsulated in canonical format over different transmission media. CFI is used to ensure compatibility between Ethernet and token ring networks. | The value 0 indicates that the MAC address is encapsulated in canonical format The value 1 indicates that the MAC address is encapsulated in non-canonical format. The CFI field has a fixed value of 0 on Ethernet networks. |
| VID | 12 bits | VLAN ID (VID) indicates the VLAN to which a frame belongs. | VLAN IDs range from 0 to 4095. The values 0 and 4095 are reserved, and therefore valid VLAN IDs range from 1 to 4094. |
The switch uses the information contained in the VID field to identify the VLAN that a frame belongs. Broadcast frames are forwarded only in the local VLAN.
VLAN Tags in Received and Sent Frames
- Tagged frame: frame with a 4-byte VLAN tag
- Untagged frame: frame without a 4-byte VLAN tag
- User hosts, servers, hubs, and Layer 2 switches can only receive and send untagged frames.
- Switches, routers, and ACs can receive and send both tagged and untagged frames.
- Voice terminals and APs can receive and send tagged and untagged frames simultaneously.
All frames processed in a switch carry VLAN tags to improve frame processing efficiency.