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This concern applies to strict uRPF customer check in a different scenario where operators need to block forged source addresses on a single-homed customer-facing interface. For the deployed customer edge ACL anti-spoof approach, the relevant caution is instead that a prefix change requires a synchronized ACL update to avoid customer outage.

This concern applies to routed optical convergence in a different scenario where operators need to reduce layers between IP routing and transport while retaining an optical link path. For the deployed customer edge ACL anti-spoof approach, the relevant caution is instead that a prefix change requires a synchronized ACL update to avoid customer outage.

This concern applies to SRv6 core path instruction in a different scenario where operators need to steer provider packets along an IPv6 segment list without MPLS label transport. For the deployed customer edge ACL anti-spoof approach, the relevant caution is instead that a prefix change requires a synchronized ACL update to avoid customer outage.

IS-IS conditional default route instead addresses a different condition: advertise a default route only under a defined service-availability condition. It would instead require you to apply default-origination with a route policy matching the required upstream state, not the stated goal to keep IS-IS routes within an area while preserving local Level-1 forwarding.

OSPF passive customer subnet instead addresses a different condition: advertise an access subnet without accepting routing hellos from its subscribers. It would instead require you to configure the access interface as OSPF passive where appropriate, not the stated goal to keep IS-IS routes within an area while preserving local Level-1 forwarding.

OSPF duplicate router identity instead addresses a different condition: resolve unstable OSPF database state caused by two routers sharing a router ID. It would instead require you to assign unique router IDs and restore the affected adjacencies, not the stated goal to keep IS-IS routes within an area while preserving local Level-1 forwarding.

The proposed step is for IS-IS conditional default route, whose purpose is to advertise a default route only under a defined service-availability condition. It would not perform the necessary IS-IS level-1 local-area reachability operation: configure Level-1 operation and matching area identifiers for those routers.

The proposed step is for OSPF passive customer subnet, whose purpose is to advertise an access subnet without accepting routing hellos from its subscribers. It would not perform the necessary IS-IS level-1 local-area reachability operation: configure Level-1 operation and matching area identifiers for those routers.

The proposed step is for OSPF duplicate router identity, whose purpose is to resolve unstable OSPF database state caused by two routers sharing a router ID. It would not perform the necessary IS-IS level-1 local-area reachability operation: configure Level-1 operation and matching area identifiers for those routers.

The observation would verify IS-IS conditional default route under the separate requirement to advertise a default route only under a defined service-availability condition. It does not demonstrate the needed IS-IS level-1 local-area reachability result: a Level-1 adjacency establishes and local-area prefixes are present.

The observation would verify OSPF passive customer subnet under the separate requirement to advertise an access subnet without accepting routing hellos from its subscribers. It does not demonstrate the needed IS-IS level-1 local-area reachability result: a Level-1 adjacency establishes and local-area prefixes are present.

The observation would verify OSPF duplicate router identity under the separate requirement to resolve unstable OSPF database state caused by two routers sharing a router ID. It does not demonstrate the needed IS-IS level-1 local-area reachability result: a Level-1 adjacency establishes and local-area prefixes are present.

This defect belongs to IS-IS conditional default route, where the affected outcome is to advertise a default route only under a defined service-availability condition. It is not the direct failure mode identified for the IS-IS level-1 local-area reachability design in this case: the devices use different IS-IS areas and no local L1 adjacency forms.

This defect belongs to OSPF passive customer subnet, where the affected outcome is to advertise an access subnet without accepting routing hellos from its subscribers. It is not the direct failure mode identified for the IS-IS level-1 local-area reachability design in this case: the devices use different IS-IS areas and no local L1 adjacency forms.

This defect belongs to OSPF duplicate router identity, where the affected outcome is to resolve unstable OSPF database state caused by two routers sharing a router ID. It is not the direct failure mode identified for the IS-IS level-1 local-area reachability design in this case: the devices use different IS-IS areas and no local L1 adjacency forms.

This concern applies to IS-IS conditional default route in a different scenario where operators need to advertise a default route only under a defined service-availability condition. For the deployed IS-IS level-1 local-area reachability approach, the relevant caution is instead that Level-1 routing alone does not distribute routes between separate areas.

This concern applies to OSPF passive customer subnet in a different scenario where operators need to advertise an access subnet without accepting routing hellos from its subscribers. For the deployed IS-IS level-1 local-area reachability approach, the relevant caution is instead that Level-1 routing alone does not distribute routes between separate areas.

This concern applies to OSPF duplicate router identity in a different scenario where operators need to resolve unstable OSPF database state caused by two routers sharing a router ID. For the deployed IS-IS level-1 local-area reachability approach, the relevant caution is instead that Level-1 routing alone does not distribute routes between separate areas.

IS-IS single-topology coexistence instead addresses a different condition: run IPv4 and IPv6 under a common IS-IS topology when links support both. It would instead require you to configure the appropriate single-topology address-family behavior consistently, not the stated goal to connect two IS-IS areas through a common Level-2 routing backbone.

OSPF designated router election instead addresses a different condition: reduce adjacency overhead on a shared broadcast Ethernet segment. It would instead require you to choose OSPF network type and DR priorities intentionally on the multiaccess network, not the stated goal to connect two IS-IS areas through a common Level-2 routing backbone.

RPL inbound AS-path policy instead addresses a different condition: reject untrusted routes with prohibited AS-path characteristics. It would instead require you to attach the intended IOS XR route-policy to the BGP neighbor inbound address family, not the stated goal to connect two IS-IS areas through a common Level-2 routing backbone.

The proposed step is for IS-IS single-topology coexistence, whose purpose is to run IPv4 and IPv6 under a common IS-IS topology when links support both. It would not perform the necessary IS-IS level-2 backbone exchange operation: form Level-2 adjacencies across area-border routers and advertise reachability.

The proposed step is for OSPF designated router election, whose purpose is to reduce adjacency overhead on a shared broadcast Ethernet segment. It would not perform the necessary IS-IS level-2 backbone exchange operation: form Level-2 adjacencies across area-border routers and advertise reachability.

The proposed step is for RPL inbound AS-path policy, whose purpose is to reject untrusted routes with prohibited AS-path characteristics. It would not perform the necessary IS-IS level-2 backbone exchange operation: form Level-2 adjacencies across area-border routers and advertise reachability.

The observation would verify IS-IS single-topology coexistence under the separate requirement to run IPv4 and IPv6 under a common IS-IS topology when links support both. It does not demonstrate the needed IS-IS level-2 backbone exchange result: remote-area routes appear in the Level-2 database and route table.

The observation would verify OSPF designated router election under the separate requirement to reduce adjacency overhead on a shared broadcast Ethernet segment. It does not demonstrate the needed IS-IS level-2 backbone exchange result: remote-area routes appear in the Level-2 database and route table.

The observation would verify RPL inbound AS-path policy under the separate requirement to reject untrusted routes with prohibited AS-path characteristics. It does not demonstrate the needed IS-IS level-2 backbone exchange result: remote-area routes appear in the Level-2 database and route table.

This defect belongs to IS-IS single-topology coexistence, where the affected outcome is to run IPv4 and IPv6 under a common IS-IS topology when links support both. It is not the direct failure mode identified for the IS-IS level-2 backbone exchange design in this case: only Level-1 peering is enabled across routers with different area IDs.

This defect belongs to OSPF designated router election, where the affected outcome is to reduce adjacency overhead on a shared broadcast Ethernet segment. It is not the direct failure mode identified for the IS-IS level-2 backbone exchange design in this case: only Level-1 peering is enabled across routers with different area IDs.

This defect belongs to RPL inbound AS-path policy, where the affected outcome is to reject untrusted routes with prohibited AS-path characteristics. It is not the direct failure mode identified for the IS-IS level-2 backbone exchange design in this case: only Level-1 peering is enabled across routers with different area IDs.

This concern applies to IS-IS single-topology coexistence in a different scenario where operators need to run IPv4 and IPv6 under a common IS-IS topology when links support both. For the deployed IS-IS level-2 backbone exchange approach, the relevant caution is instead that inter-area reachability depends on continuous Level-2 connectivity.

This concern applies to OSPF designated router election in a different scenario where operators need to reduce adjacency overhead on a shared broadcast Ethernet segment. For the deployed IS-IS level-2 backbone exchange approach, the relevant caution is instead that inter-area reachability depends on continuous Level-2 connectivity.

This concern applies to RPL inbound AS-path policy in a different scenario where operators need to reject untrusted routes with prohibited AS-path characteristics. For the deployed IS-IS level-2 backbone exchange approach, the relevant caution is instead that inter-area reachability depends on continuous Level-2 connectivity.