canonical format indicator (802.1ad) (CFI)
In VLAN tagging, which is specified in IEEE standards802.1Q and 802.1ad, an 802.1Q header consisting of the Tag Protocol Identifier( TPID) and the Tag Control Information( TCI) is inserted in Ethernet frames.
The Canonical Format Indicator (CFI) is a 1-bit data field in the Tag Control Information (TCI) that provides compatibility between Ethernet and Token Ring. This 1-bit data field indicates whether the MAC address is in a recognized (canonical) or unrecognized (non-canonical) format. If the set bit has a 0 then it is non-canonical, if it has a 1 then it is canonical. For Ethernet switches it is always set to 0. If an Ethernet port receives a 1 as CFI information, then the Ethernet switch does not connect the tagging frame to a non-tagged port.