Spanning tree algorithm for transparent bridges: The bridges exchange special control messages to figure out a spanning tree (and to adjust it later). Each bridge has a priority number (often all the same) and an ID number. 1. The root bridge is chosen as the bridge with the highest priority number. If more than one have the same priority, then the one with the smallest ID number of these is chosen. 2. Determine the root path cost (RPC) for each port on each bridge other than the root bridge. The RPC is the least cost path for a frame to go from the root bridge to the port under consideration. Cost is based on how many network segments (LANs) must be traversed and how fast these are. If all segments are the same speed, then one just needs to count the number of segments traversed. If segment 1 is three times as fast as segment 2, segment 1 might be considered to have a cost of 1 and segment 2 a cost of 3. Once all of the RPCs are known for each bridge, the bridge chooses the port with the lowest RPC as its root port (RP). If 2 or more ports have the same RPC, the lowest numbered port is chosen as the RP. The root port is then used to receive all control messages from the root bridge to this bridge. 3. For each segment, a designated bridge is chosen as the one that gives the least cost path from this segment to the root bridge. (Cost is computed as in step 2.) If 2 or more bridges give the same lowest cost, the bridge with the smallest ID number is chosen. The port that connects this segment to the designated bridge is called the designated port (DP). Obviously, the root bridge is the designated bridge for all the segments that connect to it. All of the ports of the root bridge are designated ports. Note that a root port cannot be chosen as a designated port. 4. The root and designated ports of each bridge are set to the forwarding state. Any other ports are set to the blocking state (i.e. these ports are essentially shut down). This establishes the spanning tree.