The value of the priority field which is contained in the
first (in network byte order) four bits of the (2 octet long) Port ID.
The other octet of the Port ID is given by the value of hwMstpiPortIndex.
And step of 16. It is the priority of the port in the MSTP process,
and it can be used to determine the role of the port in the MSTP process Spanning Tree.