This read-write object selects the desired spanning tree protocol of the local BCP entity, meaningful only when bcpConfigSpanTreeControl is enabled. It actually selects which spanning-tree variant, standard IEEE 802.1D, an extended variant, or IBM source-route bridging's own scheme, runs across the bridged link, and because the two half-bridges are bridging what looks like a single LAN segment to downstream devices, mismatched variants on each end can create loop-prevention blind spots a simple link-up test would never reveal. One half-bridge running standard 802.1D while its peer is set to null, no spanning tree participation, could leave a redundant bridged topology without functioning loop protection across that link.