 After this operation, the knowledge of the port status is broadcasted through the net- work via the configuration BDUs. This information is stored in a suitable database by the switches and used to route Ethernet frames in the structure without creating loops. Moreover, when a link fails SO that no frame arrives any more, the knowledge of the network status that is diffused in all the switches allows the spanning tree to be recreated excluding the failed network, changing, as needed, the states of the switch ports. The possible STP states of a switch port can be described as follows: Blocking: A port that would cause a switching loop if activated is set by the STP in this state, no user data is sent or received, but it may go into forwarding mode if the other links in use were to fail and the spanning tree algorithm determines that the port has to be activated to create a back-up path for the traffic involved in the failure. BPDU data is still received and processed in blocking state. Listening: If a port of a switch is in the listening state the switch processes BPDUs arriving through that port and awaits possible new information that would cause it to return to the blocking state or evolve to the forwarding state. Learning: While a port in the learning state does not yet forward frames, it does learn source addresses from frames received and adds them to the switching database. Forwarding: A switch port receiving and sending data is in the forwarding state. STP still monitors incoming BPDUs that would indicate it should return to the blocking state to prevent a loop. Disabled: Not strictly part of STP, a network administrator can manually disable a port. When a new device is added to the network connecting it to a switch port, it will not immediately start to forward data. It will instead go through a number of states while it processes BPDUs and determines the topology of the network. Appendix B: Spanning Tree Protocol When a host is attached, such as a computer, printer, or server, the port will always go in