zyMstpInstancePortInstanceId 1.3.6.1.4.1.890.1.15.3.53.1.6.1.1

An integer with values ranging from 0 to (zyMstpVlanMapMaxNumberOfInstances -1) that identify a CIST/MSTI instance. The value 0 represents CIST, whereas zyMstpVlanMapMaxNumberOfInstances represents the maximum number of MSTP instances.