This read-write attribute, when enabled, rejects all non-BCMP traffic in both the upstream and downstream directions and is described as being used in a Spanning Tree implementation; default is disable. It's a specialized lockdown mode an admin would use specifically while enforcing bridge/spanning-tree control traffic on a segment, not for general traffic filtering. For example, during a controlled bridging-loop investigation, an engineer might temporarily enable stuEtherBcmpOnly on a STU to ensure only spanning-tree control frames pass while all other traffic is excluded.