Three Components of 802.1Q-tagged Ethernet Frame

ACD

802.1Q is a standard used to tag Ethernet frames in order to identify VLAN membership. The tag itself is a 32-bit field that is inserted between the source MAC address and the EtherType/length fields in the Ethernet header. The tag is composed of three components:

1. Tag Protocol Identifier (TPID): This is a 16-bit field that specifies the protocol used to identify the tag. For 802.1Q, the value is always set to 0x8100.

2. Canonical Format Indicator (CFI): This is a 1-bit field that indicates whether the frame is in canonical format (i.e. follows the IEEE 802.3 Ethernet standard) or not. A value of 0 indicates canonical format, while a value of 1 indicates non-canonical format.

3. VLAN Identifier (VID): This is a 12-bit field that identifies the VLAN to which the frame belongs. It is used to ensure that frames are only forwarded to ports that are members of the same VLAN.

In summary, the three components of the tag portion of an 802.1Q-tagged Ethernet frame are the Tag Protocol Identifier (TPID), the Canonical Format Indicator (CFI), and the VLAN Identifier (VID). The TPID is always set to 0x8100 for 802.1Q frames, the CFI is a 1-bit field indicating canonical or non-canonical format, and the VID is a 12-bit field identifying the VLAN membership. The maximum transmission unit (MTU) and frame check sequence (FCS) are not part of the tag portion of an 802.1Q-tagged Ethernet frame.