The answer is an MTU mismatch on the interface, which directly causes an incomplete LSP database in IS-IS. This occurs because IS-IS derives the maximum LSP size from the interface MTU minus a small overhead (typically 3 bytes for the LSP header). When a router with a smaller MTU, such as 1500, receives an LSP generated on a link with a larger MTU, like 4470, the LSP may be too large to store or process, leading to fragmentation or outright rejection—even though IS-IS adjacencies remain up. On the Cisco SPCOR / CCNP Service Provider Core 350-501 exam, this scenario tests your understanding of how IS-IS handles LSP propagation across mixed-MTU networks; a common trap is assuming that up adjacencies guarantee a complete database. Remember the memory tip: “MTU mismatch means LSPs don’t fit—adjacencies are up, but the database is split.”
350-501 Networking Practice Question
This 350-501 practice question tests your understanding of networking. 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.
A service provider's network core runs IS-IS as the IGP. After adding a new router, some routers have incomplete LSP databases. The new router's interfaces are up, and IS-IS adjacency is up with neighbors. What is the cause? The network has a mix of interface MTUs, with some links having MTU 1500 and others 4470.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
MTU mismatch on the interface
The correct answer is A because an MTU mismatch on the interface causes incomplete LSP databases. IS-IS uses a maximum LSP size derived from the interface MTU minus the IS-IS header overhead (typically 3 bytes for the LSP header). When a router with a smaller MTU (e.g., 1500) receives an LSP that was generated on a larger MTU link (e.g., 4470), the LSP may be too large to be stored or processed, leading to fragmentation or rejection. This results in an incomplete LSP database on some routers, even though adjacencies are up.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
MTU mismatch on the interface
Why this is correct
Smaller MTU can cause LSP fragmentation/drop, leading to incomplete databases.
Related concept
Read the scenario before looking for a memorised answer.
✗
IS-IS overload bit set on the new router
Why it's wrong here
Overload bit prevents transit traffic but does not affect LSP flooding.
✗
IS-IS LSP flooding is inhibited on the new router
Why it's wrong here
IS-IS does not have a 'flood inhibition' feature; flood blocking can be configured but is not typical.
✗
The new router's system-id is a duplicate
Why it's wrong here
Duplicate system-id would prevent adjacency formation.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that MTU mismatch only affects adjacency formation, but in IS-IS, adjacencies can form even with MTU mismatch, and the real impact is on LSP database synchronization due to LSP size constraints.
Detailed technical explanation
How to think about this question
IS-IS LSPs are padded to the maximum size allowed by the interface MTU minus the IS-IS header (typically 3 bytes for the LSP header). On a 4470 MTU link, the maximum LSP size is 4467 bytes, while on a 1500 MTU link, it is 1497 bytes. If a router on a larger MTU link generates an LSP that exceeds 1497 bytes, routers on smaller MTU links cannot store it, causing incomplete databases. This is a common issue in multi-vendor or mixed-MTU environments, and the fix is to configure 'lsp-mtu' under the IS-IS process to match the smallest MTU in the network.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: MTU mismatch on the interface — The correct answer is A because an MTU mismatch on the interface causes incomplete LSP databases. IS-IS uses a maximum LSP size derived from the interface MTU minus the IS-IS header overhead (typically 3 bytes for the LSP header). When a router with a smaller MTU (e.g., 1500) receives an LSP that was generated on a larger MTU link (e.g., 4470), the LSP may be too large to be stored or processed, leading to fragmentation or rejection. This results in an incomplete LSP database on some routers, even though adjacencies are up.
What should I do if I get this 350-501 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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