Quick Answer
The answer is Level 1 LSP and Level 2 LSP. In IS-IS, Link State PDUs (LSPs) are the fundamental building blocks of the link-state database, and they are strictly categorized by routing level: Level 1 LSPs are generated by routers to describe topology within a single area, while Level 2 LSPs are used to exchange inter-area routing information between Level 2 routers. A Level 1-2 router produces both types, but it does not create a separate "Level 1-2 LSP" as a distinct type number. On the Cisco SPCOR 350-501 exam, this concept tests your understanding of IS-IS hierarchical routing and the separation of intra-area and inter-area databases. A common trap is confusing the pseudonode LSP (used for broadcast networks) as a separate type, but it is merely a special instance of a Level 1 or Level 2 LSP. To remember: think of IS-IS LSP types like two separate maps—Level 1 is your local city map, Level 2 is the national highway map, and a Level 1-2 router simply carries both maps.
This 350-501 practice question tests your understanding of architecture. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
In IS-IS, which TWO are types of Link State PDUs (LSPs)?
Partial Sequence Numbers PDU (PSNP)
Why wrong: PSNP requests missing LSPs.
Level 2 LSP
Originated by Level 2 routers for inter-area reachability.
Hello LSP
Why wrong: Hello is a separate PDU type, not an LSP.
Complete Sequence Numbers PDU (CSNP)
Why wrong: CSNP is a different PDU type for database synchronization.
Level 1 LSP
Originated by Level 1 routers within the area.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Level 2 LSP
IS-IS has three LSP types: Level 1 LSP (by routers within an area), Level 2 LSP (by Level 2 routers), and Level 1-2 LSPs (by routers that are both). However, the standard types are Level 1 and Level 2. There is also a separate pseudonode LSP for broadcast networks, but it is not a different type number. Options C, D, E are not actual LSP types in IS-IS.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Partial Sequence Numbers PDU (PSNP)
Why it's wrong here
PSNP requests missing LSPs.
Level 2 LSP
Why this is correct
Originated by Level 2 routers for inter-area reachability.
Related concept
OSPF neighbours must agree on key parameters.
Hello LSP
Why it's wrong here
Hello is a separate PDU type, not an LSP.
Complete Sequence Numbers PDU (CSNP)
Why it's wrong here
CSNP is a different PDU type for database synchronization.
Level 1 LSP
Why this is correct
Originated by Level 1 routers within the area.
Related concept
OSPF neighbours must agree on key parameters.
Common exam traps
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
What to study next
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-501 OSPF questions on adjacency and route selection.
Architecture — study guide chapter
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FAQ
Architecture — This question tests Architecture — OSPF neighbours must agree on key parameters..
The correct answer is: Level 2 LSP — IS-IS has three LSP types: Level 1 LSP (by routers within an area), Level 2 LSP (by Level 2 routers), and Level 1-2 LSPs (by routers that are both). However, the standard types are Level 1 and Level 2. There is also a separate pseudonode LSP for broadcast networks, but it is not a different type number. Options C, D, E are not actual LSP types in IS-IS.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-501 OSPF questions on adjacency and route selection.
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 24, 2026
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