Hi ,
EMISTP mode is proprietary to Extreme Networks and is an extension of STP that allows a physicalport to belong to multiple STPDs by assigning the port to multiple VLANs.
BPDUs are sent with an 802.1Q tag having an STPD instanceIdentifier (STPD ID) in the VLAN ID field.
This encapsulation mode supports the following STPD modes of operation: 802.1D and 802.1w.