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That diagnosis is supported by a different symptom: A blocked alternate port starts forwarding after BPDUs cease, creating a loop. Here the recorded observation is: a peer connected through a legacy shared segment converges slowly despite local RSTP configuration. The applicable cause is: rapid proposal/agreement handshakes are not available across that legacy/boundary link.

That diagnosis is supported by a different symptom: A small access switch becomes VLAN100's spanning-tree root after a new switch is connected. Here the recorded observation is: a peer connected through a legacy shared segment converges slowly despite local RSTP configuration. The applicable cause is: rapid proposal/agreement handshakes are not available across that legacy/boundary link.

That diagnosis is supported by a different symptom: The interface is physically up but carries some VLANs and blocks others as designed. Here the recorded observation is: a peer connected through a legacy shared segment converges slowly despite local RSTP configuration. The applicable cause is: rapid proposal/agreement handshakes are not available across that legacy/boundary link.

That control reduces a separate risk: Root priority policy was not established, so the lowest bridge ID won unexpectedly. For the described design, the recurring risk arises from: rapid proposal/agreement handshakes are not available across that legacy/boundary link. The specific required safeguard is: validate peer protocol support and link type in rapid-convergence designs.

That control reduces a separate risk: Without loop guard the switch interprets missing BPDUs as permission to transition forwarding. For the described design, the recurring risk arises from: rapid proposal/agreement handshakes are not available across that legacy/boundary link. The specific required safeguard is: validate peer protocol support and link type in rapid-convergence designs.

That control reduces a separate risk: Per-instance STP forwarding state was confused with physical-link operability. For the described design, the recurring risk arises from: rapid proposal/agreement handshakes are not available across that legacy/boundary link. The specific required safeguard is: validate peer protocol support and link type in rapid-convergence designs.

Configuring an appropriate secondary root priority higher than primary but below ordinary access switches would not remove the inappropriate BPDU Filter configuration so STP can receive and process unexpected BPDUs on the edge port.

Enabling supported loop guard on eligible non-designated point-to-point spanning-tree ports would not remove the inappropriate BPDU Filter configuration so STP can receive and process unexpected BPDUs on the edge port.

Enabling STP root guard on the appropriate designated downstream-facing interface would not remove the inappropriate BPDU Filter configuration so STP can receive and process unexpected BPDUs on the edge port.

This test measures a different behavior: Suppress BPDUs in a lab and confirm the port enters loop-inconsistent rather than transitioning to designated forwarding. It does not establish the target outcome. For the documented fault, the direct acceptance test is: inspect the interface BPDU filter state and verify received BPDUs are visible to STP during a controlled test.

This test measures a different behavior: Plug in a host and then an unauthorized BPDU sender; verify rapid forwarding then err-disable protection. It does not establish the target outcome. For the documented fault, the direct acceptance test is: inspect the interface BPDU filter state and verify received BPDUs are visible to STP during a controlled test.

This test measures a different behavior: Disable primary in a lab and inspect the next-lowest bridge ID selection. It does not establish the target outcome. For the documented fault, the direct acceptance test is: inspect the interface BPDU filter state and verify received BPDUs are visible to STP during a controlled test.

That diagnosis is supported by a different symptom: An unauthorized unmanaged switch forms a loop through a supposedly trusted desk port. Here the recorded observation is: a loop persists after an unmanaged bridge joins two access ports because their inbound BPDUs were suppressed. The applicable cause is: bPDU Filter masked incoming STP BPDUs, preventing the switch from seeing the topology change.

That diagnosis is supported by a different symptom: A blocked alternate port starts forwarding after BPDUs cease, creating a loop. Here the recorded observation is: a loop persists after an unmanaged bridge joins two access ports because their inbound BPDUs were suppressed. The applicable cause is: bPDU Filter masked incoming STP BPDUs, preventing the switch from seeing the topology change.

That diagnosis is supported by a different symptom: An unintended access switch becomes root after the designed primary goes offline. Here the recorded observation is: a loop persists after an unmanaged bridge joins two access ports because their inbound BPDUs were suppressed. The applicable cause is: bPDU Filter masked incoming STP BPDUs, preventing the switch from seeing the topology change.

That control reduces a separate risk: The distribution-facing designated link lacks root guard for the downstream boundary. For the described design, the recurring risk arises from: bPDU Filter masked incoming STP BPDUs, preventing the switch from seeing the topology change. The specific required safeguard is: prevent casual deployment of BPDU Filter on access edges; independently review PortFast and BPDU Guard eligibility.

That control reduces a separate risk: No designated secondary root priority existed to guide the failover election. For the described design, the recurring risk arises from: bPDU Filter masked incoming STP BPDUs, preventing the switch from seeing the topology change. The specific required safeguard is: prevent casual deployment of BPDU Filter on access edges; independently review PortFast and BPDU Guard eligibility.

That control reduces a separate risk: Without loop guard the switch interprets missing BPDUs as permission to transition forwarding. For the described design, the recurring risk arises from: bPDU Filter masked incoming STP BPDUs, preventing the switch from seeing the topology change. The specific required safeguard is: prevent casual deployment of BPDU Filter on access edges; independently review PortFast and BPDU Guard eligibility.

Enabling STP root guard on the appropriate designated downstream-facing interface would not configure an appropriate secondary root priority higher than primary but below ordinary access switches.

Mapping the VLAN groups into separate MST instances and tune root priorities per instance would not configure an appropriate secondary root priority higher than primary but below ordinary access switches.

Setting an appropriate lower bridge priority on the intended root switch for that STP instance/VLAN would not configure an appropriate secondary root priority higher than primary but below ordinary access switches.

This test measures a different behavior: Present a superior BPDU and verify the port enters root-inconsistent state without replacing the desired root. It does not establish the target outcome. For the documented fault, the direct acceptance test is: disable primary in a lab and inspect the next-lowest bridge ID selection.

This test measures a different behavior: Check each instance root and link roles against the intended uplink path. It does not establish the target outcome. For the documented fault, the direct acceptance test is: disable primary in a lab and inspect the next-lowest bridge ID selection.

This test measures a different behavior: Check `show spanning-tree vlan 100` root bridge ID and root port choices across the topology. It does not establish the target outcome. For the documented fault, the direct acceptance test is: disable primary in a lab and inspect the next-lowest bridge ID selection.

That diagnosis is supported by a different symptom: Both VLAN groups follow one uplink because all VLANs were left in the same MST instance. Here the recorded observation is: an unintended access switch becomes root after the designed primary goes offline. The applicable cause is: no designated secondary root priority existed to guide the failover election.

That diagnosis is supported by a different symptom: A small access switch becomes VLAN100's spanning-tree root after a new switch is connected. Here the recorded observation is: an unintended access switch becomes root after the designed primary goes offline. The applicable cause is: no designated secondary root priority existed to guide the failover election.

That diagnosis is supported by a different symptom: A newly attached access switch advertises superior BPDUs and takes over root elections. Here the recorded observation is: an unintended access switch becomes root after the designed primary goes offline. The applicable cause is: no designated secondary root priority existed to guide the failover election.

That control reduces a separate risk: The distribution-facing designated link lacks root guard for the downstream boundary. For the described design, the recurring risk arises from: no designated secondary root priority existed to guide the failover election. The specific required safeguard is: validate spanning-tree primary and secondary roles per instance during annual failure tests.

That control reduces a separate risk: Root priority policy was not established, so the lowest bridge ID won unexpectedly. For the described design, the recurring risk arises from: no designated secondary root priority existed to guide the failover election. The specific required safeguard is: validate spanning-tree primary and secondary roles per instance during annual failure tests.

That control reduces a separate risk: There is no distinct per-group spanning-tree instance topology. For the described design, the recurring risk arises from: no designated secondary root priority existed to guide the failover election. The specific required safeguard is: validate spanning-tree primary and secondary roles per instance during annual failure tests.

Enabling supported loop guard on eligible non-designated point-to-point spanning-tree ports would not interpret per-instance RSTP/MST port roles and forwarding states.

Mapping the VLAN groups into separate MST instances and tune root priorities per instance would not interpret per-instance RSTP/MST port roles and forwarding states.

Aligning MST region name, revision and VLAN-to-instance mapping on the intended region members would not interpret per-instance RSTP/MST port roles and forwarding states.