th redundancy establishing redundant links as a backup if the primary link fails. If a network node becomes unreachable for some reason, the spanning-tree algorithm reconfigures the logical topology, reestablishing the connection by activating the standby path. B.2 STP Operation The collection of bridges in a LAN can be considered as a graph whose nodes are both bridges and LAN connections, and whose edges are the interfaces connecting the bridges to the segments. To break loops in the LAN while maintaining access to all LAN segments, the bridges collectively compute a spanning tree covering all the network nodes. The spanning tree is not necessarily a minimum cost spanning tree. A network administrator can reduce the cost of a spanning tree, if necessary, by altering some of the configuration parameters in such a way as to affect the choice of the root of the spanning tree. The spanning tree that the bridges compute using the STP can be determined using sim- ple rules based on a particular representation of the network topology as a graph where both the network nodes and links are vertices and the bridge ports connecting one node to one link in the real network are the graph links. An example is represented in Figure B.1, where a simple network constituted by three bridges is represented both through its standard topology graph (where network nodes Appendix B: Spanning Tree Protocol Bridge 1 Bridge 1 Interface 1.1 Interface1.2 Interface1.1 Interface 1.2 Link 1 Link 2 Link 1 Link 2 Link 3 ID 22 ID 10 Bridge 2 Bridge 2 Bridge 3 Bridge 3 Link 3 FIGURE B.1 Example of spanning tree graph for a simple network: (a) network conventional graph; (b) spanning tree graph. are graph vertices, while network links are graph connections) and through the spanning tree graph. The steps to construct a spanning tree covering the entire network and without any loop can be summarized as follows. Step 1: Select a Root Bridge The root bridge of the spanning tree is the bridge with the smallest (lowest) bridge ID. Each bridge has a uniq