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 DIS and pseudonode on broadcast LAN. Begin with: Inspect DIS priority, elected interface MAC tie break, pseudonode LSP and adjacency state.

Inspect SSID authentication mode, RADIUS exchange, role/VLAN assignment and authentication failure logs. This check would diagnose WLAN 802.1X authentication and NAC, but would not first isolate the described IS-IS DIS and pseudonode on broadcast LAN. Begin with: Inspect DIS priority, elected interface MAC tie break, pseudonode LSP and adjacency state.

Check metric-style configuration, TLVs, interface costs and selected SPF paths. This check would diagnose IS-IS metric-style consistency, but would not first isolate the described IS-IS DIS and pseudonode on broadcast LAN. Begin with: Inspect DIS priority, elected interface MAC tie break, pseudonode LSP and adjacency state.

Establish an approved attached L1/L2 exit and verify default inter-area forwarding for Level-1 devices. This is a legitimate response to the different issue of IS-IS L1 default to nearest attached L1L2, but it does not correct the specific IS-IS CSNP and PSNP database sync condition reported. The relevant technical distinction is: CSNP and PSNP exchanges help IS-IS identify and request missing or stale link-state packets.

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 CSNP and PSNP database sync condition reported. The relevant technical distinction is: CSNP and PSNP exchanges help IS-IS identify and request missing or stale link-state packets.

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 IS-IS CSNP and PSNP database sync condition reported. The relevant technical distinction is: CSNP and PSNP exchanges help IS-IS identify and request missing or stale link-state packets.

Affected prefixes select the planned reachable next hop with correct preference while unrelated paths remain stable. That observation would support a separate case of Route-policy next-hop and preference application, not verification of the reported IS-IS CSNP and PSNP database sync. The relevant observable outcome here is: Both peers report synchronized link-state contents and calculate the same viable topology.

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 CSNP and PSNP database sync. The relevant observable outcome here is: Both peers report synchronized link-state contents and calculate the same viable topology.

The selected external next hop matches the documented cost calculation and changes predictably under failover. That observation would support a separate case of OSPF E1 versus E2 external metric, not verification of the reported IS-IS CSNP and PSNP database sync. The relevant observable outcome here is: Both peers report synchronized link-state contents and calculate the same viable topology.

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: CSNP and PSNP exchanges help IS-IS identify and request missing or stale link-state packets.

An SD-WAN overlay can select among heterogeneous underlay links using measured application path quality. This explains SD-WAN underlay and application steering, but not why the issue in this scenario occurs. The specific mechanism is: CSNP and PSNP exchanges help IS-IS identify and request missing or stale link-state packets.

Selective IS-IS route leaking can import chosen Level-2 reachability into a Level-1 area. This explains IS-IS selective L2-to-L1 route leaking, but not why the issue in this scenario occurs. The specific mechanism is: CSNP and PSNP exchanges help IS-IS identify and request missing or stale link-state packets.

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: A healthy adjacency without a consistent LSDB may produce transient wrong paths.

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: A healthy adjacency without a consistent LSDB may produce transient wrong paths.

Forcing external redistribution into a stub without redesigning its type creates unsupported expectations. This consequence is associated more directly with OSPF stub area external LSA constraint. For the stated conditions the immediate exposure is: A healthy adjacency without a consistent LSDB may produce transient wrong paths.

Inspect peer reflect-client settings, received and advertised routes and RR topology. This check would diagnose BGP route-reflector client scoping, but would not first isolate the described IS-IS CSNP and PSNP database sync. Begin with: Compare received CSNP, PSNP and LSP records and inspect network filters for protocol frames.

Inspect route communities on received paths and the community-filter and route-policy match results. This check would diagnose BGP community filtering in routing control, but would not first isolate the described IS-IS CSNP and PSNP database sync. Begin with: Compare received CSNP, PSNP and LSP records and inspect network filters for protocol frames.

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 CSNP and PSNP database sync. Begin with: Compare received CSNP, PSNP and LSP records and inspect network filters for protocol frames.

Restore an appropriate contiguous Level-2 IS-IS inter-area backbone path and verify level participation. This is a legitimate response to the different issue of IS-IS level-2 backbone continuity, but it does not correct the specific IS-IS CSNP and PSNP database sync condition reported. The relevant technical distinction is: CSNP and PSNP exchanges help IS-IS identify and request missing or stale link-state packets.

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 CSNP and PSNP database sync condition reported. The relevant technical distinction is: CSNP and PSNP exchanges help IS-IS identify and request missing or stale link-state packets.

Repair reliable OSPF LSA request and update exchange before depending on the new peer for routing. This is a legitimate response to the different issue of OSPF LSDB synchronization failure, but it does not correct the specific IS-IS CSNP and PSNP database sync condition reported. The relevant technical distinction is: CSNP and PSNP exchanges help IS-IS identify and request missing or stale link-state packets.

The Level-1 area learns its required specifics while unrelated Level-2 routes stay outside its LSDB. That observation would support a separate case of IS-IS selective L2-to-L1 route leaking, not verification of the reported IS-IS CSNP and PSNP database sync. The relevant observable outcome here is: Both peers report synchronized link-state contents and calculate the same viable topology.

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 CSNP and PSNP database sync. The relevant observable outcome here is: Both peers report synchronized link-state contents and calculate the same viable topology.

The peer session reaches Established and eligible routes exchange under the agreed AS relationship. That observation would support a separate case of BGP peer autonomous-system mismatch, not verification of the reported IS-IS CSNP and PSNP database sync. The relevant observable outcome here is: Both peers report synchronized link-state contents and calculate the same viable topology.

High-density WLAN capacity depends on shared airtime and interference management, not signal strength alone. This explains High-density WLAN RF plan, but not why the issue in this scenario occurs. The specific mechanism is: CSNP and PSNP exchanges help IS-IS identify and request missing or stale link-state packets.

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: CSNP and PSNP exchanges help IS-IS identify and request missing or stale link-state packets.

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: CSNP and PSNP exchanges help IS-IS identify and request missing or stale link-state packets.

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: A healthy adjacency without a consistent LSDB may produce transient wrong paths.

Stripping a no-export marker can leak intended internal reachability to other networks. This consequence is associated more directly with BGP route policy community NO_EXPORT. For the stated conditions the immediate exposure is: A healthy adjacency without a consistent LSDB may produce transient wrong paths.

Treating reply traffic as an independent new session can cause overbroad inbound security exceptions. This consequence is associated more directly with Stateful firewall session inspection. For the stated conditions the immediate exposure is: A healthy adjacency without a consistent LSDB may produce transient wrong paths.

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 CSNP and PSNP database sync. Begin with: Compare received CSNP, PSNP and LSP records and inspect network filters for protocol frames.

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 CSNP and PSNP database sync. Begin with: Compare received CSNP, PSNP and LSP records and inspect network filters for protocol frames.

Compare OSPF authentication settings and discarded authentication-failure counters. This check would diagnose OSPF authentication-key mismatch, but would not first isolate the described IS-IS CSNP and PSNP database sync. Begin with: Compare received CSNP, PSNP and LSP records and inspect network filters for protocol frames.