Inspect IGMP version, receiver reports, group address and installed source list. This check would diagnose IGMPv3 source-specific membership, but would not first isolate the described PIM first-hop source registration. Begin with: Inspect PIM DR, Register and Register-Stop events, RP reachability and (S,G) state.

Inspect IGMP querier address, query intervals, group timers and receiver reports. This check would diagnose IGMP last-hop membership queries, but would not first isolate the described PIM first-hop source registration. Begin with: Inspect PIM DR, Register and Register-Stop events, RP reachability and (S,G) state.

Compare address blocks, ABR area-range settings, Type 3 LSAs and specific-prefix reachability. This check would diagnose OSPF inter-area summarization placement, but would not first isolate the described PIM first-hop source registration. Begin with: Inspect PIM DR, Register and Register-Stop events, RP reachability and (S,G) state.

Enable the intended IPv6 Router Advertisements with correct prefix information and flags for the supported addressing mode. This is a legitimate response to the different issue of IPv6 SLAAC router advertisement, but it does not correct the specific PIM-SM RPT to SPT switchover condition reported. The relevant technical distinction is: PIM-SM can initially use an RP tree and later join a source-rooted SPT for efficient delivery.

Correct the unicast or multicast RPF topology so the accepted packet arrives on the expected upstream interface. This is a legitimate response to the different issue of Multicast reverse-path forwarding, but it does not correct the specific PIM-SM RPT to SPT switchover condition reported. The relevant technical distinction is: PIM-SM can initially use an RP tree and later join a source-rooted SPT for efficient delivery.

Choose an approved external metric type and metric policy to reflect the required exit selection. This is a legitimate response to the different issue of OSPF E1 versus E2 external metric, but it does not correct the specific PIM-SM RPT to SPT switchover condition reported. The relevant technical distinction is: PIM-SM can initially use an RP tree and later join a source-rooted SPT for efficient delivery.

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 PIM-SM RPT to SPT switchover. The relevant observable outcome here is: Selected high-bandwidth groups transition to the intended (S,G) shortest paths without duplicate streams.

The last-hop router records the intended (S,G) membership and only the approved source is delivered. That observation would support a separate case of IGMPv3 source-specific membership, not verification of the reported PIM-SM RPT to SPT switchover. The relevant observable outcome here is: Selected high-bandwidth groups transition to the intended (S,G) shortest paths without duplicate streams.

The receiver obtains group traffic and multicast RPF checks succeed without forwarding loops. That observation would support a separate case of Multicast reverse-path forwarding, not verification of the reported PIM-SM RPT to SPT switchover. The relevant observable outcome here is: Selected high-bandwidth groups transition to the intended (S,G) shortest paths without duplicate streams.

Edge-port safeguards prevent unintended bridge participation where endpoint-only connections are expected. This explains RSTP edge-port BPDU protection, but not why the issue in this scenario occurs. The specific mechanism is: PIM-SM can initially use an RP tree and later join a source-rooted SPT for efficient delivery.

IGMPv3 supports source-specific INCLUDE and EXCLUDE membership information for SSM. This explains IGMPv3 source-specific membership, but not why the issue in this scenario occurs. The specific mechanism is: PIM-SM can initially use an RP tree and later join a source-rooted SPT for efficient delivery.

ACL wildcard masks and ordered rule evaluation determine which packets a filtering policy matches. This explains ACL wildcard and rule sequence, but not why the issue in this scenario occurs. The specific mechanism is: PIM-SM can initially use an RP tree and later join a source-rooted SPT for efficient delivery.

Unnecessary RP dependence or an invalid source RPF path can block SSM delivery. This consequence is associated more directly with Multicast SSM and RPF source path. For the stated conditions the immediate exposure is: Keeping all high-rate flows on a poor shared-tree path can waste core bandwidth.

Bypassing RPF understanding can cause duplicate streams or persistent multicast blackholes. This consequence is associated more directly with Multicast reverse-path forwarding. For the stated conditions the immediate exposure is: Keeping all high-rate flows on a poor shared-tree path can waste core bandwidth.

An overbroad prefix filter can propagate unauthorized specifics or conceal desired routes. This consequence is associated more directly with Prefix-list network and mask matching. For the stated conditions the immediate exposure is: Keeping all high-rate flows on a poor shared-tree path can waste core bandwidth.

Inspect snooping table, querier visibility, multicast VLAN membership and egress port counters. This check would diagnose IGMP snooping multicast fanout, but would not first isolate the described PIM-SM RPT to SPT switchover. Begin with: Inspect (*,G) and (S,G) state, SPT flags, source RPF and join/prune counters.

Inspect SNMP version, users, security level, access views, manager ACL and polling errors. This check would diagnose SNMPv3 secure network monitoring, but would not first isolate the described PIM-SM RPT to SPT switchover. Begin with: Inspect (*,G) and (S,G) state, SPT flags, source RPF and join/prune counters.

Review CSS roles, health of inter-chassis paths, redundancy alarms and test traffic during failover. This check would diagnose CSS chassis clustering and failover, but would not first isolate the described PIM-SM RPT to SPT switchover. Begin with: Inspect (*,G) and (S,G) state, SPT flags, source RPF and join/prune counters.

Restore valid alternate-port eligibility and verify the supported RSTP rapid convergence procedure. This is a legitimate response to the different issue of RSTP alternate port failover, but it does not correct the specific PIM-SM RPT to SPT switchover condition reported. The relevant technical distinction is: PIM-SM can initially use an RP tree and later join a source-rooted SPT for efficient delivery.

Correct the IS-IS area identities or redesign the intended Level-2 boundary before forming the peer. This is a legitimate response to the different issue of IS-IS level-1 area adjacency, but it does not correct the specific PIM-SM RPT to SPT switchover condition reported. The relevant technical distinction is: PIM-SM can initially use an RP tree and later join a source-rooted SPT for efficient delivery.

Enable correct IGMP snooping and validate router-port and receiver-port learning for the multicast VLAN. This is a legitimate response to the different issue of IGMP snooping multicast fanout, but it does not correct the specific PIM-SM RPT to SPT switchover condition reported. The relevant technical distinction is: PIM-SM can initially use an RP tree and later join a source-rooted SPT for efficient delivery.

A simulated forward-path failure brings BFD Down and triggers the documented alternate path promptly. That observation would support a separate case of BFD fast failure detection tied to route, not verification of the reported PIM-SM RPT to SPT switchover. The relevant observable outcome here is: Selected high-bandwidth groups transition to the intended (S,G) shortest paths without duplicate streams.

The receiver joins exactly the authorized source and the distribution tree needs no RP for SSM. That observation would support a separate case of Multicast SSM and RPF source path, not verification of the reported PIM-SM RPT to SPT switchover. The relevant observable outcome here is: Selected high-bandwidth groups transition to the intended (S,G) shortest paths without duplicate streams.

A stack-link fault produces the designed protective isolation with one coherent active forwarding system. That observation would support a separate case of Switch stacking split-brain, not verification of the reported PIM-SM RPT to SPT switchover. The relevant observable outcome here is: Selected high-bandwidth groups transition to the intended (S,G) shortest paths without duplicate streams.

Edge-port safeguards prevent unintended bridge participation where endpoint-only connections are expected. This explains RSTP edge-port BPDU protection, but not why the issue in this scenario occurs. The specific mechanism is: PIM-SM can initially use an RP tree and later join a source-rooted SPT for efficient delivery.

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: PIM-SM can initially use an RP tree and later join a source-rooted SPT for efficient delivery.

Control-protocol state is necessary but not sufficient evidence of end-to-end high availability. This explains Redundancy failover path verification, but not why the issue in this scenario occurs. The specific mechanism is: PIM-SM can initially use an RP tree and later join a source-rooted SPT for efficient delivery.

Failing to protect user ports can allow a rogue switch to disrupt campus spanning-tree topology. This consequence is associated more directly with RSTP edge-port BPDU protection. For the stated conditions the immediate exposure is: Keeping all high-rate flows on a poor shared-tree path can waste core bandwidth.

Conflicting RP mappings can strand members despite healthy IGMP membership. This consequence is associated more directly with PIM-SM rendezvous-point mismatch. For the stated conditions the immediate exposure is: Keeping all high-rate flows on a poor shared-tree path can waste core bandwidth.

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: Keeping all high-rate flows on a poor shared-tree path can waste core bandwidth.

Inspect group-to-RP mapping, PIM neighbor state and (*,G) incoming interface. This check would diagnose PIM-SM rendezvous-point mismatch, but would not first isolate the described PIM-SM RPT to SPT switchover. Begin with: Inspect (*,G) and (S,G) state, SPT flags, source RPF and join/prune counters.

Capture RA packets and inspect prefix options, A/M/O flags and client IPv6 address state. This check would diagnose IPv6 SLAAC router advertisement, but would not first isolate the described PIM-SM RPT to SPT switchover. Begin with: Inspect (*,G) and (S,G) state, SPT flags, source RPF and join/prune counters.

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 PIM-SM RPT to SPT switchover. Begin with: Inspect (*,G) and (S,G) state, SPT flags, source RPF and join/prune counters.