5.2 STP Processes
5.2.1 Three steps of initial STP convergence
This section considers the algorithm that the Spanning-Tree Protocol uses to initially converge on a loop-free logical topology. Although STP has many facets, the initial convergence can be broken down into three simple steps:
  1. Elect one root bridge.
  2. Elect root ports.
  3. Elect designated ports.

When the network first starts, all the bridges are flooding the network with a mix of BPDU information. However, the bridges immediately begin applying the four-step decision sequence discussed in the previous section. This allows the bridges to hone in on the set of BPDUs that form a single spanning tree for the entire network. 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 rather like a wheel, with the root bridge as the hub and loop-free active paths (spokes) radiating outward. In a steady-state network, BPDUs flow from the root bridge outward along these loop-free spokes to every segment in the network.

After the network has converged on a loop-free active topology, utilizing this three-step process, additional changes are handled using the topology change process.

The network being used in the Figure to illustrate STP convergence consists of three switches connected in a looped configuration. Each switch has been assigned a fictitious MAC address that corresponds to the name of the device (for example, Cat-A uses MAC address AA-AA-AA-AA-AA-AA).