Disabling dispute guard

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 45, 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:

1. Port A1 can only receive BPDUs and cannot send BPDUs to Port B1.

2. Port B1 does not receive BPDUs from Port A1 for a certain period of time.

3. Device B determines itself as the root bridge.

4. Port B1 sends its BPDUs to Port A1.

5. Port A1 determines the received BPDUs are inferior to its own BPDUs. A dispute is detected.

6. Dispute guard is triggered and blocks Port A1 to prevent a loop.

However, dispute guard might disrupt the network connectivity. You can disable dispute guard to avoid connectivity loss in VLAN networks. As shown in Figure 46, the spanning tree feature is disabled on Device B and enabled on Device A and device C. Device B transparently transmits BPDUs.

Device C cannot receive superior BPDUs of VLAN 1 from Device A because Port B1 of Device B is configured to deny packets of VLAN 1. Device C determines itself as the root bridge after a certain period of time. Then, Port C1 sends an inferior BPDU of VLAN 100 to Device A.

When Device A receives the inferior BPDU, dispute guard blocks Port A1, which causes traffic interruption. To ensure service continuity, you can disable dispute guard on Device A to prevent the link from being blocked.

Disabling dispute guard might cause loops. Make sure the network is loop-free when you disable dispute guard.

To disable dispute guard: