No software changes are needed hosts. By definition, only the bridges that belong to the spanning tree are used. The path between any two hosts may not be the optimal path. An optimal path may traverse a bridge that is not part of the spanning tree and cannot be used. Broadcast and multicast frames must be flooded in all cases.
Vendors also offer bridges that connect LANs separated by fiber or phone links.
Hosts treat frames sent locally differently from those sent through bridges. Conceptually, the sending host specifies a road map saying which bridges the frame must go through to reach its destination. The numbers must be unique and are set by the network administrator. Each frame carries a source route listing the path the frame is to take. A discovery frame includes space for each bridge to add its number to the recorded path.