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Some relevant terms:
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Tagging - The act of putting 802.1Q VLAN information into the header of a packet.
-
Untagging - The act of stripping 802.1Q VLAN information out of the packet header.
■ 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)
Preamble
Destination
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
Source
VLAN TAG
Address
6 bytes
4 bytes
Length/E. type
Data
E. type
Tag
Length/E. type
Priority
CFI
VLAN ID
CFI
VLAN ID (VID)
1 bits
12 bits
2 bytes
Ethernet
Data
Type
2 bytes
46-1500 bytes
Original Ethernet
Old CRC
Data
New CRC
66
User's Manual of GT-915A
FCS
4 bytes
New Tagged Packet
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