A string of octets containing one bit per MST instances
on this trunk port. The first octet corresponds to MST
instances of 0 through 7; the second octet to MST instances
8 through 15; etc. The most significant bit of each octet
corresponds to the lowest MST instance value in that octet.
The number of bits for this mib object will be determined
by the value of stpxMSTMaxInstanceNumber.
For each instance, if it is preferred on this trunk port,
then the bit corresponding to that instance is set to '1'.