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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
0 123
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
CFIUser Priority
3 bits 1 bit 12 bits
4
Adding an IEEE 802.1 Tag
Length/EType
Dest. Src. Data Old CRC
ETypeDest. Src. Data New CRCTa g
Priority VLAN ID
Original Ethernet Packet
New Tagged Packet