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This option proposes: Distinguish level-1 area routing from level-2 backbone-style interarea routing when troubleshooting a missing destination. It addresses the different topic IS-IS level one versus level two reachability, not the directly tested IS-IS area addresses and adjacency prerequisites failure in the stated incident (A provider technician enables level-2 routing on one side of a point-to-point link while the other side stays level-1 only and then blames IPv4 address reachability.).

This option proposes: Diagnose an unexpected OSPF intra-area path by comparing cumulative interface costs through the shortest-path tree. It addresses the different topic OSPF SPF calculation and interface cost changes, not the directly tested IS-IS area addresses and adjacency prerequisites failure in the stated incident (A provider technician enables level-2 routing on one side of a point-to-point link while the other side stays level-1 only and then blames IPv4 address reachability.).

This option proposes: Trace IS-IS link-state synchronization through LSP sequence numbers and summary or acknowledgment PDUs when databases differ. It addresses the different topic IS-IS LSP synchronization using CSNP and PSNP, not the directly tested IS-IS area addresses and adjacency prerequisites failure in the stated incident (A provider technician enables level-2 routing on one side of a point-to-point link while the other side stays level-1 only and then blames IPv4 address reachability.).

This option proposes: The IS-IS designated intermediate system generates pseudonode LSP information for a broadcast LAN, and IS-IS does not create an OSPF-style backup designated intermediate system for the same segment. It addresses the different topic IS-IS designated intermediate system and pseudonode, not the directly tested IS-IS area addresses and adjacency prerequisites failure in the stated incident (A provider technician enables level-2 routing on one side of a point-to-point link while the other side stays level-1 only and then blames IPv4 address reachability.).

This option proposes: IKE negotiates security associations and authenticates peers, while IPsec ESP carries encrypted or integrity-protected traffic as configured; matching peer identities, proposals and traffic selectors are needed for an effective tunnel. It addresses the different topic IPsec VPN IKE negotiation versus ESP data channel, not the directly tested IS-IS area addresses and adjacency prerequisites failure in the stated incident (A provider technician enables level-2 routing on one side of a point-to-point link while the other side stays level-1 only and then blames IPv4 address reachability.).

This option proposes: IS-IS floods LSPs and uses Complete Sequence Number PDUs and Partial Sequence Number PDUs to summarize holdings, request missing LSPs and acknowledge entries depending on circuit behavior. It addresses the different topic IS-IS LSP synchronization using CSNP and PSNP, not the directly tested IS-IS area addresses and adjacency prerequisites failure in the stated incident (A provider technician enables level-2 routing on one side of a point-to-point link while the other side stays level-1 only and then blames IPv4 address reachability.).

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 IS-IS area addresses and adjacency prerequisites failure in the stated incident (A provider technician enables level-2 routing on one side of a point-to-point link while the other side stays level-1 only and then blames IPv4 address reachability.).

This option proposes: Inspect all routers' area flags, ABR default LSA advertisement, permitted LSA types and whether an ASBR is incorrectly placed in the area. It addresses the different topic Stub and totally stub area restrictions, not the directly tested IS-IS area addresses and adjacency prerequisites failure in the stated incident (A provider technician enables level-2 routing on one side of a point-to-point link while the other side stays level-1 only and then blames IPv4 address reachability.).

This option proposes: Examine the level-1 and level-2 routing tables, leak policy, LSP overload flags and route next hops before modifying metric preferences. It addresses the different topic IS-IS routing leakage and overload bit, not the directly tested IS-IS area addresses and adjacency prerequisites failure in the stated incident (A provider technician enables level-2 routing on one side of a point-to-point link while the other side stays level-1 only and then blames IPv4 address reachability.).

That option concerns IS-IS LSP synchronization using CSNP and PSNP. That follows from the targeted Recommended action: Inspect compatible IS-IS NET area IDs, interface levels and authentication before changing a routing process after adjacency failure.

That option concerns Multicast reverse path forwarding checks. That follows from the targeted Recommended action: Inspect compatible IS-IS NET area IDs, interface levels and authentication before changing a routing process after adjacency failure.

This option proposes: Area routes appear only where explicitly required and transit path selection avoids a router intentionally marked overloaded. It addresses the different topic IS-IS routing leakage and overload bit, not the directly tested IS-IS area addresses and adjacency prerequisites failure in the stated incident (A provider technician enables level-2 routing on one side of a point-to-point link while the other side stays level-1 only and then blames IPv4 address reachability.).

This option proposes: Validate a wireless user VLAN pool and its DHCP scope availability when client addressing becomes uneven or fails under density. It addresses the different topic WLAN VLAN pools and DHCP address scopes, not the directly tested IS-IS area addresses and adjacency prerequisites failure in the stated incident (A provider technician enables level-2 routing on one side of a point-to-point link while the other side stays level-1 only and then blames IPv4 address reachability.).

This option proposes: Trace ACL rule matching and interface direction before broadening a security filter that drops legitimate traffic. It addresses the different topic ACL traffic filter placement and rule order, not the directly tested IS-IS area addresses and adjacency prerequisites failure in the stated incident (A provider technician enables level-2 routing on one side of a point-to-point link while the other side stays level-1 only and then blames IPv4 address reachability.).

This option proposes: Compare area addresses, supported router levels, L1 and L2 link-state databases and the attached-bit/default-route behavior toward external areas. It addresses the different topic IS-IS level one versus level two reachability, not the directly tested IS-IS area addresses and adjacency prerequisites failure in the stated incident (A provider technician enables level-2 routing on one side of a point-to-point link while the other side stays level-1 only and then blames IPv4 address reachability.).

This option proposes: Inspect compatible IS-IS NET area IDs, interface levels and authentication before changing a routing process after adjacency failure. It addresses the different topic IS-IS area addresses and adjacency prerequisites, not the directly tested IS-IS designated intermediate system and pseudonode failure in the stated incident (A datacenter access VLAN has four IS-IS routers, and a reviewer incorrectly expects to see separate DR and BDR states in the IS-IS database.).

This option proposes: Trace IS-IS link-state synchronization through LSP sequence numbers and summary or acknowledgment PDUs when databases differ. It addresses the different topic IS-IS LSP synchronization using CSNP and PSNP, not the directly tested IS-IS designated intermediate system and pseudonode failure in the stated incident (A datacenter access VLAN has four IS-IS routers, and a reviewer incorrectly expects to see separate DR and BDR states in the IS-IS database.).

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 IS-IS designated intermediate system and pseudonode failure in the stated incident (A datacenter access VLAN has four IS-IS routers, and a reviewer incorrectly expects to see separate DR and BDR states in the IS-IS database.).

This option proposes: NAT rules change observed packet addresses under defined conditions, and a permitted security rule alone does not create a suitable translation mapping or solve upstream return routing. It addresses the different topic Huawei firewall NAT and address translation context, not the directly tested IS-IS designated intermediate system and pseudonode failure in the stated incident (A datacenter access VLAN has four IS-IS routers, and a reviewer incorrectly expects to see separate DR and BDR states in the IS-IS database.).

This option proposes: IS-IS floods LSPs and uses Complete Sequence Number PDUs and Partial Sequence Number PDUs to summarize holdings, request missing LSPs and acknowledge entries depending on circuit behavior. It addresses the different topic IS-IS LSP synchronization using CSNP and PSNP, not the directly tested IS-IS designated intermediate system and pseudonode failure in the stated incident (A datacenter access VLAN has four IS-IS routers, and a reviewer incorrectly expects to see separate DR and BDR states in the IS-IS database.).

This option proposes: In IS-IS, level-1 routers exchange area-local routes while level-2 routers connect areas; an L1/L2 router can provide an appropriate exit and the attached indication informs L1 routing behavior. It addresses the different topic IS-IS level one versus level two reachability, not the directly tested IS-IS designated intermediate system and pseudonode failure in the stated incident (A datacenter access VLAN has four IS-IS routers, and a reviewer incorrectly expects to see separate DR and BDR states in the IS-IS database.).

This option proposes: Compare the best route, recursive next-hop resolution, forwarding entry and actual egress interface for the same destination prefix. It addresses the different topic Network forwarding planes RIB versus FIB, not the directly tested IS-IS designated intermediate system and pseudonode failure in the stated incident (A datacenter access VLAN has four IS-IS routers, and a reviewer incorrectly expects to see separate DR and BDR states in the IS-IS database.).

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 IS-IS designated intermediate system and pseudonode failure in the stated incident (A datacenter access VLAN has four IS-IS routers, and a reviewer incorrectly expects to see separate DR and BDR states in the IS-IS database.).

This option proposes: Examine the level-1 and level-2 routing tables, leak policy, LSP overload flags and route next hops before modifying metric preferences. It addresses the different topic IS-IS routing leakage and overload bit, not the directly tested IS-IS designated intermediate system and pseudonode failure in the stated incident (A datacenter access VLAN has four IS-IS routers, and a reviewer incorrectly expects to see separate DR and BDR states in the IS-IS database.).

That option concerns IS-IS level one versus level two reachability. That follows from the targeted action: Explain the DIS role on broadcast IS-IS circuits and why it differs from an OSPF DR/BDR election.

That option concerns Route redistribution feedback loops and route tags. That follows from the targeted action: Explain the DIS role on broadcast IS-IS circuits and why it differs from an OSPF DR/BDR election.

That option concerns IS-IS routing leakage and overload bit. That follows from the targeted action: Explain the DIS role on broadcast IS-IS circuits and why it differs from an OSPF DR/BDR election.

This option proposes: Assume a small office access switch hierarchy and a multi-tenant data-center EVPN fabric are interchangeable designs merely because both carry Ethernet frames. It addresses the different topic Campus design versus data center fabric operational boundaries, not the directly tested IS-IS designated intermediate system and pseudonode failure in the stated incident (A datacenter access VLAN has four IS-IS routers, and a reviewer incorrectly expects to see separate DR and BDR states in the IS-IS database.).

This option proposes: Treat every PSNP as an ordinary IPv4 route announcement and ignore the link-state database sequence-number evidence. It addresses the different topic IS-IS LSP synchronization using CSNP and PSNP, not the directly tested IS-IS designated intermediate system and pseudonode failure in the stated incident (A datacenter access VLAN has four IS-IS routers, and a reviewer incorrectly expects to see separate DR and BDR states in the IS-IS database.).

This option proposes: Assume an IS-IS level-1-only router automatically has complete routes to all other areas without an L1/L2 exit or explicit reachability. It addresses the different topic IS-IS level one versus level two reachability, not the directly tested IS-IS designated intermediate system and pseudonode failure in the stated incident (A datacenter access VLAN has four IS-IS routers, and a reviewer incorrectly expects to see separate DR and BDR states in the IS-IS database.).

This option proposes: Check IS-IS level-1-to-level-2 route leaking and the overload bit before selecting a device as a transit path. It addresses the different topic IS-IS routing leakage and overload bit, not the directly tested IS-IS designated intermediate system and pseudonode failure in the stated incident (A network operator changes interface DIS priority during maintenance and needs to identify which pseudonode LSPs should change on that LAN.).

This option proposes: Inspect compatible IS-IS NET area IDs, interface levels and authentication before changing a routing process after adjacency failure. It addresses the different topic IS-IS area addresses and adjacency prerequisites, not the directly tested IS-IS designated intermediate system and pseudonode failure in the stated incident (A network operator changes interface DIS priority during maintenance and needs to identify which pseudonode LSPs should change on that LAN.).

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 IS-IS designated intermediate system and pseudonode failure in the stated incident (A network operator changes interface DIS priority during maintenance and needs to identify which pseudonode LSPs should change on that LAN.).