The above rules describe one way of determining what spanning tree will be computed by the algorithm, but the rules as written require knowledge of the entire network. The have to determine the root bridge and compute the port roles (root, designated, or blocked) with only the information that they have. To ensure that each bridge has enough information, the bridges use special data called bridge protocol data units (BPDUs) to exchange information about the Spanning Tree Protocol, bridge IDs, a.