This attribute specifies the name of the bridge port that offers the lowest
cost route to the root bridge of the spanning tree topology.The attribute
rootPortName is a string of the form 'BridgePortName-i', where
BridgePortName is the abbreviation of the corresponding bridge port
and i is the instance identifier of the ProtocolPort, the bridge port is
provisioned under. For example a TransparentBridgePort provisioned
under ProtocolPort/myport would have a name TBP-myport.
If multiple instances of spanning tree are operational, then there will
exist a rootPortName value for each instance of the spanning tree
running, and is applicable to that instance alone.