Determines how this virtual port behaves when presented with a Spanning Tree Explorer frame. The value 'disabled(2)' indicates that the virtual port will not accept or send Spanning Tree Explorer packets; any STE packets received will be silently discarded. The value 'forced(3)' indicates the virtual port will always accept and propagate Spanning Tree Explorer frames. This allows a manually configured Spanning Tree for this class of packet to be configured. Note that unlike transparent bridging, this is not catastrophic to the network if there are loops. The value 'auto-span(1)' can only be returned by a bridge that both implements the Spanning Tree Protocol and has use of the protocol enabled on this virtual port. The behavior of the virtual port for Spanning Tree Explorer frames is determined by the state of vStpPortState. If the virtual port is in the 'forwarding' state, the frame will be accepted or propagated. Otherwise, it will be silently discarded.