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| 3.2 |  | STP Processes |
| 3.2.2 |  | Three steps of STP convergence |

The spanning-tree algorithm is somewhat complex, but the initial process used to converge on a loop-free topology consists of three steps:

Step 1 | Elect a Root Bridge
Step 2 | Elect Root Ports
Step 3 | Elect Designated Ports

When the network first “starts” all the bridges flood the network with a mixture of BPDU information. The bridges begin applying the four-step decision sequence discussed in the preceding topic. This allows the bridges to hone in on a set of BPDUs that enable the formation of a single spanning tree for the entire network or VLAN. (The default configuration for Catalyst switches is to run one instance of the spanning-tree algorithm per VLAN.) A single Root Bridge is elected to act as the central point of this network (Step 1). All the remaining bridges calculate a set of Root Ports (Step 2) and Designated Ports (Step 3) to build a loop-free topology. The resulting topology is a tree, with the Root Bridge as the trunk and loop-free active paths radiating out-ward as tree branches. In a steady-state network, BPDUs flow from the Root Bridge outward along these loop-free branches to every segment in the network. After the network has converged on a loop-free active topology utilizing this three-step process, changes are handled using the spanning tree topology change process. The following sections consider the three steps of STP convergence in detail.