Planet SGSW-24040 User Manual page 125

1 bit of Canonical Format Identifier (CFI - used for encapsulating Token Ring packets so they can be carried across Ethernet
backbones), and 12 bits of VLAN ID (VID). The 3 bits of user 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.
User Priority
3 bits
TCI (Tag Control Information)
2 bytes
Source
VLAN TAG
Address
6 bytes 4 bytes
Length/E. type Data
Length/E. type
E. type Tag
Priority CFI VLAN ID
User's Manual of SGSW-24040 / 24240 Series
CFI
VLAN ID (VID)
1 bits 12 bits
2 bytes
Ethernet Data
Type
2 bytes 46-1500 bytes
Original Ethernet
Old CRC
Data New CRC
104
FCS
4 bytes
New Tagged Packet