Planet WGD-800 User Manual page 44

priority are used by 802.1p. The VID 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.
The tag is inserted into the packet header making the entire packet longer by 4 octets. All of the information
originally contained in the packet is retained.
802.1Q Tag
TPID (Tag Protocol Identifier)
Preamble Destination
Address
6 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. Src. Addr.
Dest. Addr. Src. Addr.
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 allows 802.1Q VLAN to span network devices
(and indeed, the entire network – if all network devices are 802.1Q compliant).
Every physical port on a switch has a PVID. 802.1Q ports are also assigned a PVID, for use within the switch. If no
VLAN are defined on the switch, all ports are then assigned to a default VLAN with a PVID equal to 1. Untagged
packets are assigned the PVID of the port on which they were received. Forwarding decisions are based upon this
PVID, in so far as VLAN are concerned. Tagged packets are forwarded according to the VID contained within the
tag. Tagged packets are also assigned a PVID, but the PVID is not used to make packet forwarding decisions, the
VID is.
User Priority
3 bits
2 bytes
Source VLAN TAG
Address
6 bytes 4 bytes
Length/E. type Data
E. type Tag Length/E. type
Priority CFI VLAN ID
CFI VLAN ID (VID)
1 bits 12 bits
TCI (Tag Control Information)
2 bytes
Ethernet Data
Type
2 bytes 46-1517 bytes
Original Ethernet
Old CRC
Data
FCS
4 bytes
New CRC
New Tagged Packet