Frame Format
802.1Q does not actually encapsulate the original frame. Instead, for Ethernet frames, it adds a 32-bit field between the source MAC address and the EtherType/Length fields of the original frame, extending the minimum and maximum frame sizes from 64 and 1,518 bytes (octets) to 64 and 1,522 bytes (42 octet minimum applies when 802.1Q is present. When absent, 46 octet minimum applies. IEEE 802.3-2005 Clause 3.5). Two bytes are used for the tag protocol identifier (TPID), the other two bytes for tag control information (TCI). The TCI field is further divided into PCP, CFI, and VID.
| 16 bits | 3 bits | 1 bit | 12 bits |
|---|---|---|---|
| TPID | TCI | ||
| PCP | CFI | VID | |
- Tag Protocol Identifier (TPID): a 16-bit field set to a value of 0x8100 in order to identify the frame as an IEEE 802.1Q-tagged frame. This field is located at the same position as the EtherType/Length field in untagged frames, and is thus used to distinguish the frame from untagged frames.
- Tag Control Identifier (TCI)
- Priority Code Point (PCP): a 3-bit field which refers to the IEEE 802.1p priority. It indicates the frame priority level. Values are from 0 (best effort) to 7 (highest); 1 represents the lowest priority. These values can be used to prioritize different classes of traffic (voice, video, data, etc.). See also Class of Service or CoS.
- Drop Eligible (DE): a 1-bit field. May be used separately or in conjunction with PCP to indicate frames eligible to be dropped in the presence of congestion.
- VLAN Identifier (VID): a 12-bit field specifying the VLAN to which the frame belongs. The hexadecimal values of 0x000 and 0xFFF are reserved. All other values may be used as VLAN identifiers, allowing up to 4,094 VLANs. The reserved value 0x000 indicates that the frame does not belong to any VLAN; in this case, the 802.1Q tag specifies only a priority and is referred to as a priority tag. On bridges, VLAN 1 (the default VLAN ID) is often reserved for a management VLAN; this is vendor-specific.
For frames using IEEE 802.2/SNAP encapsulation with an OUI field of 00-00-00 (so that the protocol ID field in the SNAP header is an EtherType), as would be the case on LANs other than Ethernet, the EtherType value in the SNAP header is set to 0x8100 and the aforementioned extra 4 bytes are appended after the SNAP header.
Because inserting the VLAN tag changes the frame, 802.1Q encapsulation forces a recalculation of the original FCS field in the Ethernet trailer.
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. Some network devices that do not support a larger frame size will process the frame successfully but may report these anomalies as a "baby giant."
Read more about this topic: IEEE 802.1Q
Famous quotes containing the word frame:
“A cold and searching wind drives away all contagion, and nothing can withstand it but what has a virtue in it, and accordingly, whatever we meet with in cold and bleak places, as the tops of mountains, we respect for a sort of sturdy innocence, a Puritan toughness. All things beside seem to be called in for shelter, and what stays out must be part of the original frame of the universe, and of such valor as God himself.”
—Henry David Thoreau (1817–1862)