0
Octets
1
2 3
4
Destination Address (6 octets)Destination Address (6 octets)
Source Address (6 octets)Source Address (6 octets)
EtherType = 0x8100EtherType = 0x8100
Tag Control InformationTag Control Information
MAC Length/TypeMAC Length/Type
Begining of DataBegining of Data
IEEE 802.1Q TagIEEE 802.1Q Tag
User PriorityUser Priority
CFI
VLAN ID (VID) (12 bits)
VLAN ID (VID) (12 bits)
3 bits3 bits
1 bit1 bit
12 bits12 bits
Cyclic Redundancy Check (4 octets)Cyclic Redundancy Check (4 octets)
The EtherType and VLAN ID are inserted after the MAC source address, but
before the original EtherType/Length or Logical Link Control. Because the packet
is now 1 bit longer than it was originally, the cyclic redundancy check (CRC) must
be recalculated.
Dest. Src.
Length/EType
Data
Dest.
Src.
EType
Tag
Length/EType Data
Priority
VLAN IDVLAN ID
Adding an IEEE 802.1Q TagAdding an IEEE 802.1Q Tag
Old
CRC
Old
CRC
New
CRC
New
CRC
Orginal Ethernet
Packet
Orginal Ethernet
Packet
New Tagged
Packet
New Tagged
Packet
Port VLAN ID
Packets that are tagged (are carrying the 802.1Q VID information) can be
transmitted from one 802.1Q compliant network device to another with the VLAN
information intact. This enables 802.1Q VLANs to span network devices (and
indeed, the entire network, if all network devices are 802.1Q compliant).
Not all network devices are 802.1Q compliant. These devices are referred to as
tag-unaware. 802.1Q devices are referred to as tag-aware.
Chapter 4. Switch management and operating concepts 27
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