Planet GSW-1602SF User Manual page 62

802.1Q VLAN Tags
The figure below shows the 802.1Q VLAN tag. There are four additional octets inserted after the source MAC address.
Their presence is indicated by a value of 0x8100 in the Ether Type field. When a packet's Ether Type field is equal to
0x8100, the packet carries the IEEE 802.1Q/802.1p tag. The tag is contained in the following two octets and consists of 3
bits of user priority, 1 bit of Canonical Format Identifier (CFI - used for encapsulating Token Ring packets so they can be
carried across Ethernet backbones), and 12 bits of VLAN ID (VID). The 3 bits of user priority are used by 802.1p. The VID
is the VLAN identifier and is used by the 802.1Q standard. Because the VID is 12 bits long, 4094 unique VLAN can be
identified.
The tag is inserted into the packet header making the entire packet longer by 4 octets. All of the information originally
contained in the packet is retained.
802.1Q Tag
TPID (Tag Protocol Identifier)
Destination
Preamble
Address
6 bytes
The Ether Type and VLAN ID are inserted after the MAC source address, but before the original Ether Type/Length or
Logical Link Control. Because the packet is now a bit longer than it was originally, the Cyclic Redundancy Check (CRC)
must be recalculated.
Adding an IEEE802.1Q Tag
Dest. Addr. Src. Addr.
Dest. Addr. Src. Addr.
User Priority
3 bits
TCI (Tag Control Information)
2 bytes 2 bytes
Source
VLAN TAG
Address
6 bytes 4 bytes
Length/E. type Data
E. type Tag
Length/E. type
Priority CFI VLAN ID
User's Manual of GSW-1602SF / GSW-2404SF
CFI VLAN ID (VID)
1 bits 12 bits
Ethernet
Data
Type
2 bytes 46-1517 bytes
Original Ethernet
Old CRC
Data New CRC
-56-
FCS
4 bytes
New Tagged Packet