This read-write value specifies which Spanning Tree protocol variant runs for the default Spanning Tree instance, per its RSStpProtocolVersion syntax; the source DESCRIPTION warns changing it causes the existing spanning tree to lose convergence. An admin sets it to select the STP variant (such as classic STP, RSTP, or a fast proprietary variant) the switch uses, understanding the change will force a reconvergence event. For example, an admin standardizing on Rapid Spanning Tree across the network would change this setting deliberately during a maintenance window, given the expected convergence disruption.