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| 2.6 |  | VLAN Identification |
| 2.6.3 |  | The IEEE 802.1Q protocol |

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 perform 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.

The IEEE 802.1Q header contains the following:

- A 4-byte tag header containing a tag protocol identifier (TPID) and tag control information (TCI)
- A 2-byte TPID with a fixed value of 0x8100 that 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

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. Network devices that do not support a larger frame size will process the frame successfully but may report these anomalies as a "baby giant."