tag control information (802.1ad) (TCI)
For VLAN tagging according to IEEE 802.1ad and 802.1ah, a 4- byte tagging field is inserted into the Ethernet frame. It consists of two 2-byte data fields each, the Tag Protocol Identifier( TPID) and the Tag Control Information (TCI).
The TCI data field, in turn, is composed of the 3-bit-long Priority Code Point( PCP) data field, the 1-bit Canonical Format Indicator( CFI) field and the 12-bit-long VLAN Identifier( VID) data field.