Figure 6- 38. IEEE 802.1Q Tag
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 a bit longer than it was originally, the Cyclic Redundancy Check (CRC) must be recalculated.
Figure 6- 39.Adding an IEEE 802.1Q Tag
56
Allied Telesyn AT-9724TS High-Density Layer 3 Stackable Gigabit Ethernet Switch
Octets
0
1
2
3
IEEE 802.1Q Tag
Destination Address (6 Octets)
Cyclic Redundancy Check (4 Octets)
VLAN ID (VID)
Source Address (6 Octets)
Ether Type = 0x8100
Tag Control Information
MAC Length/Type
Beginning of Data
CFI
User Priority
3 bits
1 bit
12 bits
4
Adding an IEEE 802.1 Tag
Length/EType
Dest.
Src.
Data
Old CRC
EType
Dest.
Src.
Data
New CRC
Tag
Priority
VLAN ID
Original Ethernet Packet
New Tagged Packet