Compare (S,G) membership, source route, upstream PIM neighbor and RPF outcome. This check would diagnose Multicast SSM and RPF source path, but would not first isolate the described MSTP VLAN-to-instance mapping. Begin with: Compare MST configuration digests, VLAN maps, revisions and CIST boundary state.

Inspect root bridge ID, path cost, port roles and current bridge priorities. This check would diagnose RSTP root bridge location, but would not first isolate the described MSTP VLAN-to-instance mapping. Begin with: Compare MST configuration digests, VLAN maps, revisions and CIST boundary state.

Inspect route-policy node IDs, if-match criteria, apply clauses and hit counts. This check would diagnose Route-policy permit and deny node order, but would not first isolate the described MSTP VLAN-to-instance mapping. Begin with: Compare MST configuration digests, VLAN maps, revisions and CIST boundary state.

Apply appropriate RSTP edge-port and BPDU protection policies while preserving legitimate uplink roles. This is a legitimate response to the different issue of RSTP edge-port BPDU protection, but it does not correct the specific MSTP region mismatch on trunks condition reported. The relevant technical distinction is: MSTP recognizes region boundaries through region name, revision and VLAN mapping digest.

Use a supported IGMPv3 source-filtering receiver configuration or documented SSM mapping where applicable. This is a legitimate response to the different issue of IGMPv3 source-specific membership, but it does not correct the specific MSTP region mismatch on trunks condition reported. The relevant technical distinction is: MSTP recognizes region boundaries through region name, revision and VLAN mapping digest.

Set reviewed RSTP bridge priorities so an appropriate resilient core device remains root. This is a legitimate response to the different issue of RSTP root bridge location, but it does not correct the specific MSTP region mismatch on trunks condition reported. The relevant technical distinction is: MSTP recognizes region boundaries through region name, revision and VLAN mapping digest.

The NSSA carries the Type 7 route and eligible outside areas learn its translated external prefix. That observation would support a separate case of OSPF NSSA type-7 translation, not verification of the reported MSTP region mismatch on trunks. The relevant observable outcome here is: The intended region shares coherent instance topology and redundant paths remain loop-free.

An unauthorized bridge is blocked or the port enters its defined protected state without changing the core root. That observation would support a separate case of RSTP edge-port BPDU protection, not verification of the reported MSTP region mismatch on trunks. The relevant observable outcome here is: The intended region shares coherent instance topology and redundant paths remain loop-free.

Root-port selection and link topology match the designed forwarding tree without a Layer 2 loop. That observation would support a separate case of RSTP root bridge location, not verification of the reported MSTP region mismatch on trunks. The relevant observable outcome here is: The intended region shares coherent instance topology and redundant paths remain loop-free.

IGMP uses group membership messages and queries to maintain last-hop receiver state. This explains IGMP last-hop membership queries, but not why the issue in this scenario occurs. The specific mechanism is: MSTP recognizes region boundaries through region name, revision and VLAN mapping digest.

In AC+Fit AP WLAN architectures the AC centrally manages AP configuration over CAPWAP. This explains WLAN AC and Fit AP CAPWAP, but not why the issue in this scenario occurs. The specific mechanism is: MSTP recognizes region boundaries through region name, revision and VLAN mapping digest.

Stacking combines multiple devices into a logical control plane and requires protection against split brain. This explains Switch stacking split-brain, but not why the issue in this scenario occurs. The specific mechanism is: MSTP recognizes region boundaries through region name, revision and VLAN mapping digest.

Any-source joins cannot always represent source-specific subscriptions and may allow unwanted streams. This consequence is associated more directly with IGMPv3 source-specific membership. For the stated conditions the immediate exposure is: An accidental boundary can change port roles and send traffic over unintended links.

Simultaneous active stack fragments can duplicate gateways or destabilize Layer 2 forwarding. This consequence is associated more directly with Switch stacking split-brain. For the stated conditions the immediate exposure is: An accidental boundary can change port roles and send traffic over unintended links.

An accidental edge root can cause suboptimal paths and widespread topology changes. This consequence is associated more directly with RSTP root bridge location. For the stated conditions the immediate exposure is: An accidental boundary can change port roles and send traffic over unintended links.

Inspect stack role, heartbeat, split-brain detection state and inter-member link health. This check would diagnose Switch stacking split-brain, but would not first isolate the described MSTP region mismatch on trunks. Begin with: Inspect region digest, MSTI states, instance root priorities and boundary-port indicators.

Review ingress marks, traffic policy counters, queue occupancy and congestion test results. This check would diagnose QoS DSCP remarking and trust, but would not first isolate the described MSTP region mismatch on trunks. Begin with: Inspect region digest, MSTI states, instance root priorities and boundary-port indicators.

Inspect port roles, proposal/agreement exchanges, physical link state and failover logs. This check would diagnose RSTP alternate port failover, but would not first isolate the described MSTP region mismatch on trunks. Begin with: Inspect region digest, MSTI states, instance root priorities and boundary-port indicators.

Align the MST region name, revision and VLAN-to-instance map across participating switches. This is a legitimate response to the different issue of MSTP VLAN-to-instance mapping, but it does not correct the specific MSTP region mismatch on trunks condition reported. The relevant technical distinction is: MSTP recognizes region boundaries through region name, revision and VLAN mapping digest.

Evaluate eligible candidate paths and select the shorter AS_PATH when earlier BGP preference criteria do not decide. This is a legitimate response to the different issue of BGP AS_PATH length egress choice, but it does not correct the specific MSTP region mismatch on trunks condition reported. The relevant technical distinction is: MSTP recognizes region boundaries through region name, revision and VLAN mapping digest.

Configure an exact supported policy-based route match and viable next hop for the authorized traffic class. This is a legitimate response to the different issue of Policy-based routing for approved application, but it does not correct the specific MSTP region mismatch on trunks condition reported. The relevant technical distinction is: MSTP recognizes region boundaries through region name, revision and VLAN mapping digest.

An unauthorized bridge is blocked or the port enters its defined protected state without changing the core root. That observation would support a separate case of RSTP edge-port BPDU protection, not verification of the reported MSTP region mismatch on trunks. The relevant observable outcome here is: The intended region shares coherent instance topology and redundant paths remain loop-free.

The chosen route matches the documented origin tie-break sequence and other path attributes. That observation would support a separate case of BGP origin attribute tie break, not verification of the reported MSTP region mismatch on trunks. The relevant observable outcome here is: The intended region shares coherent instance topology and redundant paths remain loop-free.

An authorized failover moves the alternate link to forwarding without creating a loop. That observation would support a separate case of RSTP alternate port failover, not verification of the reported MSTP region mismatch on trunks. The relevant observable outcome here is: The intended region shares coherent instance topology and redundant paths remain loop-free.

Multicast routers use reverse-path forwarding checks to accept packets arriving from the expected source-facing direction. This explains Multicast reverse-path forwarding, but not why the issue in this scenario occurs. The specific mechanism is: MSTP recognizes region boundaries through region name, revision and VLAN mapping digest.

An MST region's identity includes consistent region parameters and VLAN-to-instance mapping. This explains MSTP VLAN-to-instance mapping, but not why the issue in this scenario occurs. The specific mechanism is: MSTP recognizes region boundaries through region name, revision and VLAN mapping digest.

OSPF ABRs convey eligible inter-area reachability through network summary LSAs. This explains OSPF type-3 inter-area summaries, but not why the issue in this scenario occurs. The specific mechanism is: MSTP recognizes region boundaries through region name, revision and VLAN mapping digest.

An accidental edge root can cause suboptimal paths and widespread topology changes. This consequence is associated more directly with RSTP root bridge location. For the stated conditions the immediate exposure is: An accidental boundary can change port roles and send traffic over unintended links.

Assuming a Hello neighbor alone provides a complete topology can create inconsistent routing decisions. This consequence is associated more directly with OSPF LSDB synchronization failure. For the stated conditions the immediate exposure is: An accidental boundary can change port roles and send traffic over unintended links.

Simultaneous active stack fragments can duplicate gateways or destabilize Layer 2 forwarding. This consequence is associated more directly with Switch stacking split-brain. For the stated conditions the immediate exposure is: An accidental boundary can change port roles and send traffic over unintended links.

Inspect edge-port flags, BPDU-protection events, root changes and MAC-move logs. This check would diagnose RSTP edge-port BPDU protection, but would not first isolate the described MSTP region mismatch on trunks. Begin with: Inspect region digest, MSTI states, instance root priorities and boundary-port indicators.

Correlate BFD and VRRP events with actual packet captures, route tables and application checks. This check would diagnose Redundancy failover path verification, but would not first isolate the described MSTP region mismatch on trunks. Begin with: Inspect region digest, MSTI states, instance root priorities and boundary-port indicators.

Inspect ACL rule numbers, match counts, attachment direction and permitted/denied test flows. This check would diagnose ACL packet filter rule order and direction, but would not first isolate the described MSTP region mismatch on trunks. Begin with: Inspect region digest, MSTI states, instance root priorities and boundary-port indicators.