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 metric-style consistency. Begin with: Check metric-style configuration, TLVs, interface costs and selected SPF paths.

Inspect import policy, level-specific LSP advertisements and the Level-1 route table. This check would diagnose IS-IS selective L2-to-L1 route leaking, but would not first isolate the described IS-IS metric-style consistency. Begin with: Check metric-style configuration, TLVs, interface costs and selected SPF paths.

Compare interface MTU, DD packet MTU fields, and OSPF neighbor transition logs. This check would diagnose OSPF MTU stalls ExStart, but would not first isolate the described IS-IS metric-style consistency. Begin with: Check metric-style configuration, TLVs, interface costs and selected SPF paths.

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 IS-IS selective L2-to-L1 route leaking condition reported. The relevant technical distinction is: Selective IS-IS route leaking can import chosen Level-2 reachability into a Level-1 area.

Configure the actual authorized remote AS consistently with the peer's BGP OPEN identity. This is a legitimate response to the different issue of BGP peer autonomous-system mismatch, but it does not correct the specific IS-IS selective L2-to-L1 route leaking condition reported. The relevant technical distinction is: Selective IS-IS route leaking can import chosen Level-2 reachability into a Level-1 area.

Mark imported routes and deny reimport of already redistributed origins with controlled route tags. This is a legitimate response to the different issue of Redistribution route tag loop avoidance, but it does not correct the specific IS-IS selective L2-to-L1 route leaking condition reported. The relevant technical distinction is: Selective IS-IS route leaking can import chosen Level-2 reachability into a Level-1 area.

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 selective L2-to-L1 route leaking. The relevant observable outcome here is: The Level-1 area learns its required specifics while unrelated Level-2 routes stay outside its LSDB.

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 selective L2-to-L1 route leaking. The relevant observable outcome here is: The Level-1 area learns its required specifics while unrelated Level-2 routes stay outside its LSDB.

One appropriate DIS per active level advertises the LAN pseudonode and the LSDB remains consistent. That observation would support a separate case of IS-IS DIS and pseudonode on broadcast LAN, not verification of the reported IS-IS selective L2-to-L1 route leaking. The relevant observable outcome here is: The Level-1 area learns its required specifics while unrelated Level-2 routes stay outside its LSDB.

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: Selective IS-IS route leaking can import chosen Level-2 reachability into a Level-1 area.

BGP NO_EXPORT community constrains external AS propagation when carried and honored. This explains BGP route policy community NO_EXPORT, but not why the issue in this scenario occurs. The specific mechanism is: Selective IS-IS route leaking can import chosen Level-2 reachability into a Level-1 area.

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: Selective IS-IS route leaking can import chosen Level-2 reachability into a Level-1 area.

Changing inbound MED may not correct the enterprise's outbound egress preference. This consequence is associated more directly with BGP local preference internal egress. For the stated conditions the immediate exposure is: Unrestricted leaking can expand LSDB size and introduce unintended route preference.

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: Unrestricted leaking can expand LSDB size and introduce unintended route preference.

Incorrect DIS assumptions can cause inconsistent troubleshooting and churn in shared-segment LSPs. This consequence is associated more directly with IS-IS DIS and pseudonode on broadcast LAN. For the stated conditions the immediate exposure is: Unrestricted leaking can expand LSDB size and introduce unintended route preference.

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 selective L2-to-L1 route leaking. Begin with: Inspect import policy, level-specific LSP advertisements and the Level-1 route table.

Inspect source system ID, LSP sequence, remaining lifetime and interface flap history. This check would diagnose IS-IS LSP sequence and aging, but would not first isolate the described IS-IS selective L2-to-L1 route leaking. Begin with: Inspect import policy, level-specific LSP advertisements and the Level-1 route table.

Compare received CSNP, PSNP and LSP records and inspect network filters for protocol frames. This check would diagnose IS-IS CSNP and PSNP database sync, but would not first isolate the described IS-IS selective L2-to-L1 route leaking. Begin with: Inspect import policy, level-specific LSP advertisements and the Level-1 route table.

Configure supported inter-subnet or inter-AC roaming design appropriate to the mobility domain. This is a legitimate response to the different issue of Campus WLAN layer-3 roaming, but it does not correct the specific IS-IS selective L2-to-L1 route leaking condition reported. The relevant technical distinction is: Selective IS-IS route leaking can import chosen Level-2 reachability into a Level-1 area.

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 selective L2-to-L1 route leaking condition reported. The relevant technical distinction is: Selective IS-IS route leaking can import chosen Level-2 reachability into a Level-1 area.

Tune approved OSPF link costs to represent the intended path preference and re-evaluate SPF. This is a legitimate response to the different issue of OSPF SPF path-cost comparison, but it does not correct the specific IS-IS selective L2-to-L1 route leaking condition reported. The relevant technical distinction is: Selective IS-IS route leaking can import chosen Level-2 reachability into a Level-1 area.

A supported Level-1 adjacency forms only between appropriate same-area neighbors and routes appear correctly. That observation would support a separate case of IS-IS level-1 area adjacency, not verification of the reported IS-IS selective L2-to-L1 route leaking. The relevant observable outcome here is: The Level-1 area learns its required specifics while unrelated Level-2 routes stay outside its LSDB.

The shortest-path calculation selects the intended cost path and the backup remains viable. That observation would support a separate case of OSPF SPF path-cost comparison, not verification of the reported IS-IS selective L2-to-L1 route leaking. The relevant observable outcome here is: The Level-1 area learns its required specifics while unrelated Level-2 routes stay outside its LSDB.

OSPF authentication errors cease and the expected peer adjacency returns without disabling authentication. That observation would support a separate case of OSPF authentication-key mismatch, not verification of the reported IS-IS selective L2-to-L1 route leaking. The relevant observable outcome here is: The Level-1 area learns its required specifics while unrelated Level-2 routes stay outside its LSDB.

IS-IS metric encoding capability influences which costs can be advertised and interpreted. This explains IS-IS metric-style consistency, but not why the issue in this scenario occurs. The specific mechanism is: Selective IS-IS route leaking can import chosen Level-2 reachability into a Level-1 area.

Level-1 IS-IS routers commonly use the attached-bit indication to select an inter-area exit. This explains IS-IS L1 default to nearest attached L1L2, but not why the issue in this scenario occurs. The specific mechanism is: Selective IS-IS route leaking can import chosen Level-2 reachability into a Level-1 area.

OSPF database description negotiation can fail when peer interface MTU expectations do not match. This explains OSPF MTU stalls ExStart, but not why the issue in this scenario occurs. The specific mechanism is: Selective IS-IS route leaking can import chosen Level-2 reachability into a Level-1 area.

Untested clustering can leave a single point of failure despite two physical chassis. This consequence is associated more directly with CSS chassis clustering and failover. For the stated conditions the immediate exposure is: Unrestricted leaking can expand LSDB size and introduce unintended route preference.

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: Unrestricted leaking can expand LSDB size and introduce unintended route preference.

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: Unrestricted leaking can expand LSDB size and introduce unintended route preference.

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 selective L2-to-L1 route leaking. Begin with: Inspect import policy, level-specific LSP advertisements and the Level-1 route table.

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 selective L2-to-L1 route leaking. Begin with: Inspect import policy, level-specific LSP advertisements and the Level-1 route table.

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 IS-IS selective L2-to-L1 route leaking. Begin with: Inspect import policy, level-specific LSP advertisements and the Level-1 route table.