2.1.i [ii] STP dispute, STP bridge assurance
When a designated port detects a conflict, it keeps its role, but reverts to a discarding state because disrupting connectivity in case of inconsistency is preferable to starting a bridging loop.
The figure below illustrates a unidirectional link failure that typically creates a bridging loop. Switch A is the root bridge, and its BPDUs are lost on the link leading to switch B. The 802.1w-standard BPDUs include the role and state of the sending port. With this information, switch A can detect that switch B does not react to the superior BPDUs that it sends and that switch B is the designated, not root port. As a result, switch A blocks (or keeps blocking ) its port, which prevents the bridging loop.
You can use Bridge Assurance to protect against certain problems that can cause bridging loops in the network. Specifically, you can use Bridge Assurance to protect against a unidirectional link failure and a device that continues to forward data traffic when it is no longer running the spanning tree algorithm. Bridge Assurance is enabled by default and can only be disabled globally. In addition to that, Bridge Assurance can only be enabled on spanning tree network ports that are point-to-point links. Both ends of the link must have Bridge Assurance enabled. With Bridge Assurance enabled, BPDUs are sent out on all operational network ports in both directions, including alternate and backup ports, for each hello time period. If the port does not receive a BPDU for a specified period, the port moves into the blocking state and is not used in the root port calculation. Once that port receives a BPDU, it resumes the normal spanning tree transitions.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1929-1936). . Kindle Edition.