IEEE 802.1Q VLAN frame format
Original Ethernet Frame Format
Ethernet Frames on a tagged port can include a VLAN
Label Field Name Size Description
PA Preamble 7 bytes Used to synchronize traffic between nodes
SF Start Frame Delimiter 1 bytes Marks the beginning of the header
DA Destination Address 6 bytes The MAC address of the next/final hop
SA Source Address 6 bytes The MAC address of the source
TPI Tag Protocol Identifier 2 bytes Indicates this frame uses 802.1p or Q tags – set to “8100” in the standard
P User Priority 3 bits Indicates 802.1p priority level 0-7 (CoS)
CFI Canonical Format Indicator 1 bit Indicates if the MAC addresses are in canonical format (bit ordering
information) – Ethernet uses “0” / different in Token Ring
VLAN ID VLAN Identifier (VID) 12 bits Indicates which VLAN this frame belongs to (1-4094)
T/L Type/Length Field 2 bytes Ethernet II “type” or 802.3 “length” information
Payload Payload 48 - 1500
bytes
User data or higher layer protocol information
FCS Frame Check Sequence 4 bytes Error checking on the frame’s contents – also known as “CRC” (Cyclical
Redundancy Check)
User
Priority (P)
CFI
VLAN ID (VID)
to identify 4094 possible VLANs
81 00
TPI
PA SFD DA
SA
TL
Data Bytes 46 - 1500 Byte
FCS IFG
PA SFD DA
SA
TPI
VLAN
TL
Data Bytes 46 - 1500 Byte
FCS IFG
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