STP BPDUs are used as probes to learn about a network topology. When the switches participating in STP converge on a common and consistent loop-free topology, BPDUs still must be sent by the Root Bridge and must be relayed by every other switch in the STP domain. The STP topology's integrity then depends on a continuous and regular flow of BPDUs from the root.
What happens if a switch doesn't receive BPDUs in a timely manner or when it doesn't receive any? The switch can view that condition as acceptable—perhaps an upstream switch or an upstream link is dead. In that case, the topology must have changed, so blocked ports eventually can be unblocked again.
However, if the absence of BPDUs is actually a mistake and BPDUs are not being received even though there is no topology change, bridging loops easily can form.
Cisco has added two STP features that help detect or prevent the unexpected loss of BPDUs:
■ Unidirectional Link Detection (UDLD)
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