- The software checks the consistency of the port role and state in the received BPDUs to detect unidirectional link failures that could cause a loop.
- When a designated port detects a conflict (receives an inferior BPDU), it keeps its role, but reverts to a discarding state.
- Rapid PVST+ (802.1w) and MST BPDUs include the role and state of the sending port. With this information, switch A (root) can detect that switch B does not react to its superior BPDUs and that switch B is the designated port, not root port. As a result, switch A blocks the port.
- The block is shown as an STP dispute.
STP Bridge Assurance
- Bridge Assurance is enabled by default and can only be disabled globally (available on Cisco IOS Release 12.2(33)SXI and later).
- Only enabled on STP network ports that are point-to-point links.
- Both ends of the link must have Bridge Assurance enabled.
- If the device on one side of the link has Bridge Assurance enabled and the device on the other side either does not support Bridge Assurance or does not have this feature enabled, the connecting port is blocked.
- With Bridge Assurance enabled, BPDUs are sent out on all operational network ports, 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 an inconsistent state (blocking). and is not used in the root port calculation.
- Once that port receives a BPDU, it resumes the normal spanning tree transitions.
- Recommended to enable Bridge Assurance throughout the network.
DocumentationSpanning-Tree Dispute Mechanism
Understanding Bridge Assurance