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28 Intel® Blade Server Ethernet Switch Module IXM5414E
IEEE 802.1Q VLAN tags
The following illustration shows the 802.1Q VLAN tag. Four additional octets are inserted between
the source MAC address and the packet’s EtherType field. Their presence is indicated by a value of
0x8100 in the two bytes following the MAC address, in the VLAN tag’s EtherType field, indicating
that the packet carries an IEEE 802.1Q/802.1p tag. The tag is contained in the following 2 octets and
consists of 3 bits of user priority, 1 bit of Canonical Format Identifier (CFI) and 12 bits of VLAN ID
(VID). The 3 bits of user priority are used according to the protocols defined in IEEE 802.1p (now
part of IEEE 802.1D). The VID is the VLAN identifier and its use is defined by the 802.1Q standard.
Because the VID is 12 bits long, 4094 unique VLANs can be identified.
The tag is inserted into the packet header, increasing the length of the entire packet by 4 octets. All
of the information that was originally contained in the packet is retained.
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 longer than it was
originally, the cyclic redundancy check (CRC) must be recalculated.
0
Octets
1
2 3
4
Destination Address (6 octets)
Destination Address (6 octets)
Source Address (6 octets)
Source Address (6 octets)
EtherType = 0x8100
EtherType = 0x8100
Tag Control Information
Tag Control Information
MAC Length/Type
MAC Length/Type
Begining of Data
Begining of Data
IEEE 802.1Q Tag
IEEE 802.1Q Tag
User Priority
User Priority
CFI
VLAN ID (VID) (12 bits)
VLAN ID (VID) (12 bits)
3 bits
3 bits
1 bit
1 bit
12 bits
12 bits
Cyclic Redundancy Check (4 octets)
Cyclic Redundancy Check (4 octets)