User’s Manual of WSD-800 / WGSD-910/WGSD-1022C
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802.1Q Tag
User Priority CFI VLAN ID (VID)
3 bits 1 bits 12 bits
TPID (Tag Protocol Identifier) TCI (Tag Control Information)
2 bytes 2 bytes
Preamble Destination
Address
Source
Address
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. Src. Addr. Length/E. type Data Old CRC
Dest. Addr. Src. Addr. E. type Tag Length/E. type Data New CRC
Priority CFI VLAN ID
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.
Tag-aware switches must keep a table to relate PVID within the switch to VID on the network. The switch will
compare the VID of a packet to be transmitted to the VID of the port that is to transmit the packet. If the two VID
are different the switch will drop the packet. Because of the existence of the PVID for untagged packets and the
VID for tagged packets, tag-aware and tag-unaware network devices can coexist on the same network.
A switch port can have only one PVID, but can have as many VID as the switch has memory in its VLAN table to
store them.
Original Ethernet
New Tagged Packet
