This attribute determines how this bridge port will react when presented
with a Spanning Tree Explore frame.
If disabled, the Spanning Tree Explore frame is discarded regardless of
the value of stpPortState.
If the value of this attribute is forced, the Spanning Tree Explore frame
is forwarded regardless of the value of stpPortState.
autoSpan uses the stpPortState value to decide whether or not Spanning
Tree Explore frames should be forwarded out this bridge port. If the port
is in the forwarding state, the frame is received and transmitted
otherwise the frame is discarded.