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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 level one versus level two reachability failure in the stated incident (A service provider changes a transit router from level-1-2 to level-1 only and sees destinations in other IS-IS areas become unreachable.).

This option proposes: Explain the DIS role on broadcast IS-IS circuits and why it differs from an OSPF DR/BDR election. It addresses the different topic IS-IS designated intermediate system and pseudonode, not the directly tested IS-IS level one versus level two reachability failure in the stated incident (A service provider changes a transit router from level-1-2 to level-1 only and sees destinations in other IS-IS areas become unreachable.).

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 IS-IS level one versus level two reachability failure in the stated incident (A service provider changes a transit router from level-1-2 to level-1 only and sees destinations in other IS-IS areas become unreachable.).

This option proposes: MED is an optional nontransitive BGP attribute, with lower values typically favored when compared under applicable rules; it is often compared for routes from the same neighboring AS unless policy changes this behavior. It addresses the different topic BGP MED interpretation between neighboring autonomous systems, not the directly tested IS-IS level one versus level two reachability failure in the stated incident (A service provider changes a transit router from level-1-2 to level-1 only and sees destinations in other IS-IS areas become unreachable.).

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 level one versus level two reachability failure in the stated incident (A service provider changes a transit router from level-1-2 to level-1 only and sees destinations in other IS-IS areas become unreachable.).

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 level one versus level two reachability failure in the stated incident (A service provider changes a transit router from level-1-2 to level-1 only and sees destinations in other IS-IS areas become unreachable.).

This option proposes: Inspect the client's DHCP Discover/Offer exchange, relay agent address, server route, configured pool and interface addressing. It addresses the different topic DHCP relay across a routed user VLAN, not the directly tested IS-IS level one versus level two reachability failure in the stated incident (A service provider changes a transit router from level-1-2 to level-1 only and sees destinations in other IS-IS areas become unreachable.).

This option proposes: Inspect LAN IIH priorities, elected DIS system ID, pseudonode LSPs and neighbor topology before changing interface priority. It addresses the different topic IS-IS designated intermediate system and pseudonode, not the directly tested IS-IS level one versus level two reachability failure in the stated incident (A service provider changes a transit router from level-1-2 to level-1 only and sees destinations in other IS-IS areas become unreachable.).

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 level one versus level two reachability failure in the stated incident (A service provider changes a transit router from level-1-2 to level-1 only and sees destinations in other IS-IS areas become unreachable.).

That option concerns Policy-based routing versus destination routing. That follows from the targeted Recommended action: Distinguish level-1 area routing from level-2 backbone-style interarea routing when troubleshooting a missing destination.

That option concerns IS-IS LSP synchronization using CSNP and PSNP. That follows from the targeted Recommended action: Distinguish level-1 area routing from level-2 backbone-style interarea routing when troubleshooting a missing destination.

That option concerns IS-IS area addresses and adjacency prerequisites. That follows from the targeted Recommended action: Distinguish level-1 area routing from level-2 backbone-style interarea routing when troubleshooting a missing destination.

This option proposes: Ignore a router's active IS-IS overload bit and force all area-to-area traffic through it because its loopback is reachable. It addresses the different topic IS-IS routing leakage and overload bit, not the directly tested IS-IS level one versus level two reachability failure in the stated incident (A service provider changes a transit router from level-1-2 to level-1 only and sees destinations in other IS-IS areas become unreachable.).

This option proposes: Tell an area-border engineer that all OSPF routers must flood identical type 1 LSAs throughout every area to guarantee inter-area connectivity. It addresses the different topic OSPF router network and summary LSA scope, not the directly tested IS-IS level one versus level two reachability failure in the stated incident (A service provider changes a transit router from level-1-2 to level-1 only and sees destinations in other IS-IS areas become unreachable.).

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 IS-IS level one versus level two reachability failure in the stated incident (A service provider changes a transit router from level-1-2 to level-1 only and sees destinations in other IS-IS areas become unreachable.).

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 (Two neighboring branch routers share an Ethernet VLAN but their IS-IS level-1 adjacency fails after one NET was moved to another area.).

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 (Two neighboring branch routers share an Ethernet VLAN but their IS-IS level-1 adjacency fails after one NET was moved to another area.).

This option proposes: Review broadcast-network designated-router election based on interface priority and router ID, noting that a stable election does not automatically preempt. It addresses the different topic OSPF broadcast DR and BDR election, not the directly tested IS-IS area addresses and adjacency prerequisites failure in the stated incident (Two neighboring branch routers share an Ethernet VLAN but their IS-IS level-1 adjacency fails after one NET was moved to another area.).

This option proposes: Type 1 router and type 2 network LSAs describe area-local topology, whereas ABRs create type 3 summary LSAs for destinations in other areas; a lack of appropriate LSAs affects SPF reachability. It addresses the different topic OSPF router network and summary LSA scope, not the directly tested IS-IS area addresses and adjacency prerequisites failure in the stated incident (Two neighboring branch routers share an Ethernet VLAN but their IS-IS level-1 adjacency fails after one NET was moved to another area.).

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 area addresses and adjacency prerequisites failure in the stated incident (Two neighboring branch routers share an Ethernet VLAN but their IS-IS level-1 adjacency fails after one NET was moved to another area.).

This option proposes: IS-IS LSP control flags and route-leaking policy determine appropriate reachability; an overload indication can steer other routers away from using an overloaded node as transit while preserving selected directly attached reachability. 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 (Two neighboring branch routers share an Ethernet VLAN but their IS-IS level-1 adjacency fails after one NET was moved to another area.).

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 (Two neighboring branch routers share an Ethernet VLAN but their IS-IS level-1 adjacency fails after one NET was moved to another area.).

This option proposes: Check whether the interface should form neighbors, its authentication mode, passive state, advertised prefix and neighboring device visibility. It addresses the different topic OSPF interface security and passive network advertisement, not the directly tested IS-IS area addresses and adjacency prerequisites failure in the stated incident (Two neighboring branch routers share an Ethernet VLAN but their IS-IS level-1 adjacency fails after one NET was moved to another area.).

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 (Two neighboring branch routers share an Ethernet VLAN but their IS-IS level-1 adjacency fails after one NET was moved to another area.).

That option concerns IS-IS designated intermediate system and pseudonode. That follows from the targeted 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 (Two neighboring branch routers share an Ethernet VLAN but their IS-IS level-1 adjacency fails after one NET was moved to another area.).

That option concerns Route redistribution feedback loops and route tags. That follows from the targeted action: Inspect compatible IS-IS NET area IDs, interface levels and authentication before changing a routing process after adjacency failure.

This option proposes: Trace PIM-SM receiver joins toward an RP and subsequent shortest-path behavior when investigating delayed or suboptimal multicast. It addresses the different topic PIM sparse mode RP shared tree and shortest path transition, not the directly tested IS-IS area addresses and adjacency prerequisites failure in the stated incident (Two neighboring branch routers share an Ethernet VLAN but their IS-IS level-1 adjacency fails after one NET was moved to another area.).

This option proposes: Use a carefully scoped Huawei IP prefix list to match the intended route ranges and prefix lengths before applying an import or export policy. It addresses the different topic IP prefix filtering and exact route-match conditions, not the directly tested IS-IS area addresses and adjacency prerequisites failure in the stated incident (Two neighboring branch routers share an Ethernet VLAN but their IS-IS level-1 adjacency fails after one NET was moved to another area.).

This option proposes: Inspect LAN IIH priorities, elected DIS system ID, pseudonode LSPs and neighbor topology before changing interface priority. 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 (Two neighboring branch routers share an Ethernet VLAN but their IS-IS level-1 adjacency fails after one NET was moved to another area.).

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.).