Planet IGS-801T [51/120] Q vlan tags

User’s Manual of IGS-801M

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802.1Q VLAN Tags

s 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

IEEE 802.1Q/802.1p tag. The tag is contained in the following two octets and consists of 3

anonical Format Identifier (CFI - used for encapsulating Token Ring packets so they can be

VLAN ID (VID) D

The tag is inserted into the packet header making the entire packet longer by 4 octets. All of the information originally

The figure below show

0x8100, the packet carries the

bits of user priority, 1 bit of C

carried across Ethernet backbones), and 12 bits of . The 3 bits of user priority are used by 802.1p. The VI

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.

contained in the packet is retained.

802.1Q Tag

VLAN ID (VID) User Priority CFI

3 bits 1 bits 12 bits

TPID (Tag Protocol Identifier) TCI (Tag Control Information)

2 bytes 2 bytes

Preamble

Address Address

Destination Source

VLAN TAG

Ethernet

Type

Data FCS

6 bytes 6 bytes 4 bytes 2 bytes 46-1517 bytes 4 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 Data Old CRC . Src. Addr. Length/E. type

Dest. Addr. Src. Addr. E. type Tag Length/E. type Data New CRC

Original Ethernet

New Tagged Packet

Priority CFI

VLAN ID