Standard for Virtual Bridged Local Area Networks (IEEE 802.1Q)

Standard for Virtual Bridged Local Area Networks (IEEE 802.1Q)
Unlike ISL, which uses an external tagging process and encapsulates a frame with a new ISL
encapsulation, 802.1Q uses an internal tagging process by modifying the existing internal Ethernet
frame. To access both links and trunk links, the frame looks as if it is just a standard Ethernet
frame because it is not encapsulated with VLAN information. The VLAN information is added to
a field within the frame itself.
Like ISL, the purpose of 802.1Q is to carry the traffic of more than one subnet down a single
cable. 802.1Q tags the frame in a standard VLAN format, which allows for the VLAN implementations
of multiple vendors. The standard tag allows for an open architecture and standard
services for VLANs and a standard for protocols in the provision of these services. Because adding
VLAN information to a frame affects the frame length, two committees were created to deal
with this issue: 802.3ac and 802.1Q.
The VLAN frame format defined in both the 802.1Q and 802.3ac is a four-byte field that
is inserted between the original Ethernet frame Source address field and the Type or Length
field. The CRC of the frame must be recomputed whenever the VLAN information is
inserted or removed from the frame. The Ethernet frame size can now be up to 1522 bytes
if a tag is inserted.
The VLAN Tag Protocol Identifier (TPID) is globally assigned and uses an EtherType field
value of 0x81-00. The Tag Control Information (TCI) is a 16-bit value and has three fields contained
within:
User Priority A three-bit field used to assign up to eight layers of priority. The highest priority
is 0, and the lowest is 7 (specified in 802.1Q).
Canonical Format Indicator (CFI) A one-bit field that is always a 0 if running an 802.3 frame.
This field was originally designed to be used for Token Ring VLANs, but it was never implemented
except for some proprietary Token Ring LANs.
VLAN ID (VID) The actual VLAN number that the frame is assigned upon entering the
switch (12 bits). The reserved VLAN IDs are as follows:
0x0-00 Null, or no VLAN ID, which is used when only priority information is sent
0x0-01 Default VLAN value of all switches
0x-F-FF Reserved
Because Ethernet frames can not exceed 1518 bytes, and ISL and 802.1Q frames can exceed
1518 bytes, the switch might record the frame as a baby giant frame.