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The official name for the IEEE 802.1Q
protocol is the Standard for Virtual Bridged Local-Area Networks,
and relates to the ability to carry the traffic of more than one
subnet down a single cable. The IEEE 802.1Q committee defined this
method of multiplexing VLANs in an effort to provide multivendor
VLAN support.
Both ISL and IEEE 802.1Q tagging are
explicit tagging, meaning that the frame is tagged with VLAN
information explicitly. However, while ISL uses an external tagging
process that does not modify the existing Ethernet frame, IEEE
802.1Q uses an internal tagging process that does modify the
Ethernet frame. This internal tagging process is what allows IEEE
802.1Q tagging to work on both access and trunk links, because the frame appears to be a standard Ethernet
frame.
The IEEE 802.1Q frame-tagging scheme
also has significantly less overhead than the ISL tagging method. As
opposed to the 30 bytes added by ISL, 802.1Q inserts only an
additional 4 bytes into the Ethernet frame, as shown in the Figure.
The IEEE 802.1Q header contains the
following:
- A 4-byte tag header containing a tag
protocol identifier (TPID) and tag control information (TCI)
with the following elements:
- A 2-byte TPID with a fixed value
of 0x8100. This value indicates that the frame carries the
802.1Q/802.1p tag information.
- A TCI containing the following
elements:
- Three-bit user priority
- One-bit canonical format indicator
(CFI)
- Twelve-bit VLAN identifier (VID)-Uniquely
identifies the VLAN to which the frame belongs
Note: The CFI is used in Ethernet frames to indicate the presence of a Routing Information Field (RIF) - the RIF is used in Token Ring networks to indicate the route the frame is to take through the network (source-route bridging).
The 802.1Q standard can create an
interesting scenario on the network. Recalling that the maximum size
for an Ethernet frame as specified by IEEE 802.3 is 1518 bytes, this
means that if a maximum-sized Ethernet frame gets tagged, the frame
size will be 1522 bytes, a number that violates the IEEE 802.3
standard. To resolve this issue, the 802.3 committee created a
subgroup called 802.3ac to extend the maximum Ethernet size to 1522
bytes. If you have network devices that do not support a larger
frame size, they will process the frame successfully but may report
these anomalies as "baby giant."
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