In STP or RSTP, it describes link status according to port STP
steady status. In MSTP it describes STP steady status of the
instance which management VLAN belongs to. Either
'hgmpNTDPTopEdgeStartPort' or 'hgmpNTDPTopEdgeEndPort' STP
steady status is 'blocking', this object is 'blocking',
otherwise is 'forwarding'.
If STP or MSTP is disabled, this object is 'forwarding'.