About dispute guard
Dispute guard is enabled by default. You do not need to manually configure this feature.
Dispute guard can be triggered by unidirectional link failures. If an upstream port receives inferior BPDUs from a downstream designated port in forwarding or learning state because of a unidirectional link failure, a loop appears. Dispute guard blocks the upstream designated port to prevent the loop.
As shown in Figure 31, in normal conditions, the spanning tree calculation result is as follows:
Device A is the root bridge, and Port A1 is a designated port.
Port B1 is blocked.
When the link between Port A1 and Port B1 fails in the direction of Port A1 to Port B1 and becomes unidirectional, the following events occur:
Port A1 can only receive BPDUs and cannot send BPDUs to Port B1.
Port B1 does not receive BPDUs from Port A1 for a certain period of time.
Device B determines itself as the root bridge.
Port B1 sends its BPDUs to Port A1.
Port A1 determines the received BPDUs are inferior to its own BPDUs. A dispute is detected.
Dispute guard is triggered and blocks Port A1 to prevent a loop.
Figure 31: Dispute guard triggering scenario
