is not marked also when the raps fl frame is received by the ring connection nodes b 1 and b 2 18 and 16 connected to the sub ring 30 because the ring id information included in the raps fl frame and the ring id information stored in the ring connection nodes b 1 and b 2 18 and 16 is different the ring connection nodes b 1 and b 2 18 and 16 operate like general ring nodes namely the ring connection nodes b 1 and b 2 18 and 16 determine whether or not to erase their filtering database with reference to the erasure flag and multicast the raps fl frame in the opposite direction of the ring eventually the raps fl frame multicast from the ring connection nodes a 1 and a 2 12 and 14 is transferred to the rpl owner node s 2 17 that owns a blockage in the major ring 10 in this case the raps fl frame transferred to the rpl owner node s 2 17 is not transferred in the opposite direction any longer due to the logical blockage with reference to fig1 c because the rpl owner node s 2 17 of the major ring 10 owns a blockage it marks the blockage flag included in the received raps fl frame and multicasts the same in both directions of the ring in this case the blockage of the rpl owner.