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This option proposes: Attempt to configure an OSPF BDR role for IS-IS, assuming the DIS cannot operate until a backup DIS has been selected. It addresses the different topic IS-IS designated intermediate system and pseudonode, not the directly tested PIM sparse mode RP shared tree and shortest path transition failure in the stated incident (A PIM-SM network initially forwards through a shared tree, then switches to a lower-latency source tree, and analysts need to explain the change in (S,G) state.).

This option proposes: Assume PIM-DM always requires a rendezvous point and never initially floods multicast packets to uninterested downstream segments. It addresses the different topic PIM dense mode flooding and prune mechanisms, not the directly tested PIM sparse mode RP shared tree and shortest path transition failure in the stated incident (A PIM-SM network initially forwards through a shared tree, then switches to a lower-latency source tree, and analysts need to explain the change in (S,G) state.).

This option proposes: Assume a router must forward every packet sent to a multicast group whenever it has a unicast default route, regardless of the packet's incoming interface. It addresses the different topic Multicast reverse path forwarding checks, not the directly tested PIM sparse mode RP shared tree and shortest path transition failure in the stated incident (A PIM-SM network initially forwards through a shared tree, then switches to a lower-latency source tree, and analysts need to explain the change in (S,G) state.).

This option proposes: Validate IPv6 router advertisements, prefix information and duplicate-address detection before treating a newly generated address as usable. It addresses the different topic IPv6 SLAAC RA prefix and duplicate address detection, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (Two IPv6 hosts on one VLAN have valid global addresses but cannot exchange packets after a security ACL begins dropping Neighbor Solicitation messages.).

This option proposes: Check route-policy matching conditions, node permit/deny behavior and application direction before fixing unexpected accepted or rejected routes. It addresses the different topic Route-policy node and filtering direction, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (Two IPv6 hosts on one VLAN have valid global addresses but cannot exchange packets after a security ACL begins dropping Neighbor Solicitation messages.).

This option proposes: Apply OSPF neighbor authentication and passive-interface policy without accidentally suppressing required route advertisements. It addresses the different topic OSPF interface security and passive network advertisement, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (Two IPv6 hosts on one VLAN have valid global addresses but cannot exchange packets after a security ACL begins dropping Neighbor Solicitation messages.).

This option proposes: IS-IS uses CLNS-based network entities and level-specific hellos; level-1 neighbors normally require compatible area membership, while level-2 adjacencies have different area constraints and still need link and authentication compatibility. It addresses the different topic IS-IS area addresses and adjacency prerequisites, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (Two IPv6 hosts on one VLAN have valid global addresses but cannot exchange packets after a security ACL begins dropping Neighbor Solicitation messages.).

This option proposes: Stateless address autoconfiguration uses advertised IPv6 prefix information and local interface identifier behavior, while Duplicate Address Detection probes for conflicts before enabling the address; DHCPv6 flags influence additional configuration modes. It addresses the different topic IPv6 SLAAC RA prefix and duplicate address detection, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (Two IPv6 hosts on one VLAN have valid global addresses but cannot exchange packets after a security ACL begins dropping Neighbor Solicitation messages.).

This option proposes: A BGP network statement generally requires an appropriate matching route in the local routing table, whereas importing routes from other protocols uses an explicit redistribution mechanism; export policy can further restrict announcements. It addresses the different topic BGP route origin network and redistribution policy, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (Two IPv6 hosts on one VLAN have valid global addresses but cannot exchange packets after a security ACL begins dropping Neighbor Solicitation messages.).

This option proposes: Compare LSP identifiers, sequence numbers, aging, CSNP summaries, PSNP entries and adjacency state on the affected circuit. It addresses the different topic IS-IS LSP synchronization using CSNP and PSNP, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (Two IPv6 hosts on one VLAN have valid global addresses but cannot exchange packets after a security ACL begins dropping Neighbor Solicitation messages.).

This option proposes: Review RA source, autonomous prefix flag, valid and preferred lifetimes, DAD NS traffic, address state and default router list. It addresses the different topic IPv6 SLAAC RA prefix and duplicate address detection, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (Two IPv6 hosts on one VLAN have valid global addresses but cannot exchange packets after a security ACL begins dropping Neighbor Solicitation messages.).

This option proposes: Inspect ingress interface VPN instance, per-instance routes, next-hop resolution and authorized route import or interconnect rules. It addresses the different topic VRF traffic separation and route leaking controls, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (Two IPv6 hosts on one VLAN have valid global addresses but cannot exchange packets after a security ACL begins dropping Neighbor Solicitation messages.).

That option concerns IPv6 SLAAC RA prefix and duplicate address detection. That follows from the targeted action: Investigate IPv6 neighbor resolution using ICMPv6 Neighbor Solicitation and Neighbor Advertisement rather than IPv4 ARP assumptions.

That option concerns IS-IS routing leakage and overload bit. That follows from the targeted action: Investigate IPv6 neighbor resolution using ICMPv6 Neighbor Solicitation and Neighbor Advertisement rather than IPv4 ARP assumptions.

That option concerns VRF traffic separation and route leaking controls. That follows from the targeted action: Investigate IPv6 neighbor resolution using ICMPv6 Neighbor Solicitation and Neighbor Advertisement rather than IPv4 ARP assumptions.

This option proposes: Validate resilient AC roles and per-user access decisions when WLAN controller failure or endpoint authorization affects service. It addresses the different topic WLAN controller redundancy and NAC integration, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (Two IPv6 hosts on one VLAN have valid global addresses but cannot exchange packets after a security ACL begins dropping Neighbor Solicitation messages.).

This option proposes: Review RA source, autonomous prefix flag, valid and preferred lifetimes, DAD NS traffic, address state and default router list. It addresses the different topic IPv6 SLAAC RA prefix and duplicate address detection, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (Two IPv6 hosts on one VLAN have valid global addresses but cannot exchange packets after a security ACL begins dropping Neighbor Solicitation messages.). The better discriminating check for this case is: Inspect neighbor cache states, NS/NA exchanges, solicited-node multicast, router advertisements and link-layer addresses on the affected VLAN.

This option proposes: Assess fast roaming within and between AP coverage cells using the configured mobility relationship and authentication requirements. It addresses the different topic Wireless roaming mobility and session continuity, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (Two IPv6 hosts on one VLAN have valid global addresses but cannot exchange packets after a security ACL begins dropping Neighbor Solicitation messages.).

This option proposes: Diagnose firewall traffic by checking stateful session records and security-zone policy rather than treating every permitted ACL packet as an established connection. It addresses the different topic Stateful firewall sessions versus basic packet ACL filtering, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (An operator sees a reachable IPv4 gateway and assumes IPv6 address resolution must work despite no Neighbor Advertisement from the IPv6 router.).

This option proposes: Distinguish EVPN control-plane route information and underlay forwarding when analyzing a VXLAN-enabled fabric. It addresses the different topic MP-BGP EVPN route types and forwarding separation, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (An operator sees a reachable IPv4 gateway and assumes IPv6 address resolution must work despite no Neighbor Advertisement from the IPv6 router.).

This option proposes: Validate IPv6 router advertisements, prefix information and duplicate-address detection before treating a newly generated address as usable. It addresses the different topic IPv6 SLAAC RA prefix and duplicate address detection, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (An operator sees a reachable IPv4 gateway and assumes IPv6 address resolution must work despite no Neighbor Advertisement from the IPv6 router.).

This option proposes: Inter-area OSPF design relies on a connected backbone area; virtual links can logically connect certain disconnected backbone segments through a suitable transit area but cannot pass through a stub area. It addresses the different topic OSPF backbone connectivity and virtual link design, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (An operator sees a reachable IPv4 gateway and assumes IPv6 address resolution must work despite no Neighbor Advertisement from the IPv6 router.).

This option proposes: Stateless address autoconfiguration uses advertised IPv6 prefix information and local interface identifier behavior, while Duplicate Address Detection probes for conflicts before enabling the address; DHCPv6 flags influence additional configuration modes. It addresses the different topic IPv6 SLAAC RA prefix and duplicate address detection, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (An operator sees a reachable IPv4 gateway and assumes IPv6 address resolution must work despite no Neighbor Advertisement from the IPv6 router.).

This option proposes: Redistributing between routing protocols can create route feedback, suboptimal preference or loops unless policies identify origins, limit permitted prefixes and use suitable tagging or filtering. It addresses the different topic Route redistribution feedback loops and route tags, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (An operator sees a reachable IPv4 gateway and assumes IPv6 address resolution must work despite no Neighbor Advertisement from the IPv6 router.).

This option proposes: Review RA source, autonomous prefix flag, valid and preferred lifetimes, DAD NS traffic, address state and default router list. It addresses the different topic IPv6 SLAAC RA prefix and duplicate address detection, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (An operator sees a reachable IPv4 gateway and assumes IPv6 address resolution must work despite no Neighbor Advertisement from the IPv6 router.).

This option proposes: Map the ABRs, area membership, transit-area properties and database synchronization along the proposed virtual-link path. It addresses the different topic OSPF backbone connectivity and virtual link design, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (An operator sees a reachable IPv4 gateway and assumes IPv6 address resolution must work despite no Neighbor Advertisement from the IPv6 router.).

This option proposes: Inspect multicast routing entries, incoming interface, unicast RPF route for the source or RP and packet counters on the observed ingress. It addresses the different topic Multicast reverse path forwarding checks, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (An operator sees a reachable IPv4 gateway and assumes IPv6 address resolution must work despite no Neighbor Advertisement from the IPv6 router.).

That option concerns BGP MED interpretation between neighboring autonomous systems. That follows from the targeted Recommended action: Investigate IPv6 neighbor resolution using ICMPv6 Neighbor Solicitation and Neighbor Advertisement rather than IPv4 ARP assumptions.

That option concerns IPv6 SLAAC RA prefix and duplicate address detection. That follows from the targeted Recommended action: Investigate IPv6 neighbor resolution using ICMPv6 Neighbor Solicitation and Neighbor Advertisement rather than IPv4 ARP assumptions.

This option proposes: The selected BGP prefix has a resolvable next hop and traffic exits through the intended edge after the appropriate next-hop policy. It addresses the different topic BGP NEXT_HOP reachability and next-hop-self, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (An operator sees a reachable IPv4 gateway and assumes IPv6 address resolution must work despite no Neighbor Advertisement from the IPv6 router.). The better discriminating check for this case is: Inspect neighbor cache states, NS/NA exchanges, solicited-node multicast, router advertisements and link-layer addresses on the affected VLAN. The targeted correction is: Investigate IPv6 neighbor resolution using ICMPv6 Neighbor Solicitation and Neighbor Advertisement rather than IPv4 ARP assumptions.

This option proposes: Verify BGP network origination and import-route mechanisms before expecting a prefix to be advertised. It addresses the different topic BGP route origin network and redistribution policy, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (An operator sees a reachable IPv4 gateway and assumes IPv6 address resolution must work despite no Neighbor Advertisement from the IPv6 router.).

This option proposes: Review RA source, autonomous prefix flag, valid and preferred lifetimes, DAD NS traffic, address state and default router list. It addresses the different topic IPv6 SLAAC RA prefix and duplicate address detection, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (An operator sees a reachable IPv4 gateway and assumes IPv6 address resolution must work despite no Neighbor Advertisement from the IPv6 router.). The better discriminating check for this case is: Inspect neighbor cache states, NS/NA exchanges, solicited-node multicast, router advertisements and link-layer addresses on the affected VLAN.

This option proposes: Diagnose a multicast drop by checking reverse path forwarding against the packet's source rather than only validating the destination group. It addresses the different topic Multicast reverse path forwarding checks, not the directly tested IPv6 neighbor discovery NS NA and address resolution failure in the stated incident (An operator sees a reachable IPv4 gateway and assumes IPv6 address resolution must work despite no Neighbor Advertisement from the IPv6 router.).