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:
- Elect one root bridge.
- Elect root ports.
- 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).
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