User’s Manual of FSD-804PS
802.1Q VLAN Tags
The figure below shows 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
0x8100, the packet carries the IEEE 802.1Q/802.1p tag. The tag is contained in the following two octets and consists of 3
bits of user priority, 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.
IEEE 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 Ad-
dress
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 IEEE 802.1Q 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
New Tagged Packet
Original Ethernet
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.
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