Inspect (*,G) and (S,G) state, SPT flags, source RPF and join/prune counters. This check would diagnose PIM-SM RPT to SPT switchover, but would not first isolate the described IS-IS level-1 area adjacency. Begin with: Inspect IS-IS NETs, area IDs, level capabilities and Hello authentication on both interfaces.

Compare interface cost values, SPF results, route cost and the selected next hop. This check would diagnose OSPF SPF path-cost comparison, but would not first isolate the described IS-IS level-1 area adjacency. Begin with: Inspect IS-IS NETs, area IDs, level capabilities and Hello authentication on both interfaces.

Inspect Level-2 LSDBs, adjacency levels, attached bits and inter-area route tables. This check would diagnose IS-IS level-2 backbone continuity, but would not first isolate the described IS-IS level-1 area adjacency. Begin with: Inspect IS-IS NETs, area IDs, level capabilities and Hello authentication on both interfaces.

Configure supported matching OSPF authentication mode and keys on both authorized neighbors. This is a legitimate response to the different issue of OSPF authentication-key mismatch, but it does not correct the specific IS-IS level-2 backbone continuity condition reported. The relevant technical distinction is: IS-IS uses Level-2 routing for inter-area exchange between Level-1 domains.

Examine LSP sequence numbers and lifetime behavior and correct the root flooding or instability problem. This is a legitimate response to the different issue of IS-IS LSP sequence and aging, but it does not correct the specific IS-IS level-2 backbone continuity condition reported. The relevant technical distinction is: IS-IS uses Level-2 routing for inter-area exchange between Level-1 domains.

Apply a reviewed community-filter match in the relevant import or export route-policy. This is a legitimate response to the different issue of BGP community filtering in routing control, but it does not correct the specific IS-IS level-2 backbone continuity condition reported. The relevant technical distinction is: IS-IS uses Level-2 routing for inter-area exchange between Level-1 domains.

A simulated PBR next-hop outage selects the approved alternate without routing loops. That observation would support a separate case of PBR failed-next-hop fallback, not verification of the reported IS-IS level-2 backbone continuity. The relevant observable outcome here is: Level-2 adjacencies carry the required inter-area reachability between participating regions.

Newer LSPs supersede outdated versions and a stable LSDB yields the intended routing path. That observation would support a separate case of IS-IS LSP sequence and aging, not verification of the reported IS-IS level-2 backbone continuity. The relevant observable outcome here is: Level-2 adjacencies carry the required inter-area reachability between participating regions.

The prefix remains usable internally where authorized and is not advertised to prohibited external peers. That observation would support a separate case of BGP route policy community NO_EXPORT, not verification of the reported IS-IS level-2 backbone continuity. The relevant observable outcome here is: Level-2 adjacencies carry the required inter-area reachability between participating regions.

IS-IS Level-1 neighbor relationships require compatible area addressing while Level-2 uses broader backbone reachability. This explains IS-IS level-1 area adjacency, but not why the issue in this scenario occurs. The specific mechanism is: IS-IS uses Level-2 routing for inter-area exchange between Level-1 domains.

OSPF external route summarization is performed on an ASBR, distinct from ABR inter-area summary generation. This explains OSPF external summarization at ASBR, but not why the issue in this scenario occurs. The specific mechanism is: IS-IS uses Level-2 routing for inter-area exchange between Level-1 domains.

BGP exchanges routing control messages over an established TCP transport connection. This explains BGP TCP reachability and port, but not why the issue in this scenario occurs. The specific mechanism is: IS-IS uses Level-2 routing for inter-area exchange between Level-1 domains.

Incorrectly treating an old LSP as current can preserve outdated reachability after recovery. This consequence is associated more directly with IS-IS LSP sequence and aging. For the stated conditions the immediate exposure is: Isolated Level-1 areas cannot reach unrelated areas without a working Level-2 path.

Disabling AS loop detection to hide a policy error risks persistent routing loops. This consequence is associated more directly with BGP AS_PATH loop detection. For the stated conditions the immediate exposure is: Isolated Level-1 areas cannot reach unrelated areas without a working Level-2 path.

Adding power without channel planning can increase retries and worsen real-time service. This consequence is associated more directly with High-density WLAN RF plan. For the stated conditions the immediate exposure is: Isolated Level-1 areas cannot reach unrelated areas without a working Level-2 path.

Inspect neighbor phase, LSR retransmission lists, relevant LSAs and interface loss. This check would diagnose OSPF LSDB synchronization failure, but would not first isolate the described IS-IS level-2 backbone continuity. Begin with: Inspect Level-2 LSDBs, adjacency levels, attached bits and inter-area route tables.

Inspect IKE negotiation logs, IPsec SA status, peer identity and proxy selectors. This check would diagnose IPsec peer proposal compatibility, but would not first isolate the described IS-IS level-2 backbone continuity. Begin with: Inspect Level-2 LSDBs, adjacency levels, attached bits and inter-area route tables.

Check attached bits in LSPs, candidate L1/L2 routes and default forwarding behavior. This check would diagnose IS-IS L1 default to nearest attached L1L2, but would not first isolate the described IS-IS level-2 backbone continuity. Begin with: Inspect Level-2 LSDBs, adjacency levels, attached bits and inter-area route tables.

Place both ends of the adjacency in the intended common OSPF area and verify area design constraints. This is a legitimate response to the different issue of OSPF area-ID mismatch, but it does not correct the specific IS-IS level-2 backbone continuity condition reported. The relevant technical distinction is: IS-IS uses Level-2 routing for inter-area exchange between Level-1 domains.

Apply a narrow supported Level-2-to-Level-1 route-leak policy only for authorized service prefixes. This is a legitimate response to the different issue of IS-IS selective L2-to-L1 route leaking, but it does not correct the specific IS-IS level-2 backbone continuity condition reported. The relevant technical distinction is: IS-IS uses Level-2 routing for inter-area exchange between Level-1 domains.

Check earlier best-path attributes and prefer the supported origin order IGP, then EGP, then Incomplete where reached. This is a legitimate response to the different issue of BGP origin attribute tie break, but it does not correct the specific IS-IS level-2 backbone continuity condition reported. The relevant technical distinction is: IS-IS uses Level-2 routing for inter-area exchange between Level-1 domains.

The prefix remains usable internally where authorized and is not advertised to prohibited external peers. That observation would support a separate case of BGP route policy community NO_EXPORT, not verification of the reported IS-IS level-2 backbone continuity. The relevant observable outcome here is: Level-2 adjacencies carry the required inter-area reachability between participating regions.

Eligible areas see the intended external aggregate and services within it retain correct reachability. That observation would support a separate case of OSPF external summarization at ASBR, not verification of the reported IS-IS level-2 backbone continuity. The relevant observable outcome here is: Level-2 adjacencies carry the required inter-area reachability between participating regions.

Every device interprets intended link costs and SPF selects the documented shortest paths. That observation would support a separate case of IS-IS metric-style consistency, not verification of the reported IS-IS level-2 backbone continuity. The relevant observable outcome here is: Level-2 adjacencies carry the required inter-area reachability between participating regions.

IS-IS broadcast networks elect a DIS per level and use pseudonodes to represent LAN topology. This explains IS-IS DIS and pseudonode on broadcast LAN, but not why the issue in this scenario occurs. The specific mechanism is: IS-IS uses Level-2 routing for inter-area exchange between Level-1 domains.

OSPF neighbors must agree on relevant Hello and Dead interval parameters. This explains OSPF hello and dead timer mismatch, but not why the issue in this scenario occurs. The specific mechanism is: IS-IS uses Level-2 routing for inter-area exchange between Level-1 domains.

IS-IS identifies the freshness of LSPs using sequence numbers and lifetime fields. This explains IS-IS LSP sequence and aging, but not why the issue in this scenario occurs. The specific mechanism is: IS-IS uses Level-2 routing for inter-area exchange between Level-1 domains.

Absent or misleading attached exits can strand a local area or direct it into an invalid path. This consequence is associated more directly with IS-IS L1 default to nearest attached L1L2. For the stated conditions the immediate exposure is: Isolated Level-1 areas cannot reach unrelated areas without a working Level-2 path.

Duplicate router IDs can corrupt topology interpretation and interrupt route convergence. This consequence is associated more directly with OSPF duplicate router IDs. For the stated conditions the immediate exposure is: Isolated Level-1 areas cannot reach unrelated areas without a working Level-2 path.

Mismatched metric styles can hide prefixes or distort shortest-path selection. This consequence is associated more directly with IS-IS metric-style consistency. For the stated conditions the immediate exposure is: Isolated Level-1 areas cannot reach unrelated areas without a working Level-2 path.

Review channel utilization, RF profiles, client count, survey measurements and retries. This check would diagnose High-density WLAN RF plan, but would not first isolate the described IS-IS level-2 backbone continuity. Begin with: Inspect Level-2 LSDBs, adjacency levels, attached bits and inter-area route tables.

Inspect IS-IS NETs, area IDs, level capabilities and Hello authentication on both interfaces. This check would diagnose IS-IS level-1 area adjacency, but would not first isolate the described IS-IS level-2 backbone continuity. Begin with: Inspect Level-2 LSDBs, adjacency levels, attached bits and inter-area route tables.

Check area stub configuration on all routers, Type 3 default and any local ASBR attempts. This check would diagnose OSPF stub area external LSA constraint, but would not first isolate the described IS-IS level-2 backbone continuity. Begin with: Inspect Level-2 LSDBs, adjacency levels, attached bits and inter-area route tables.