350-501 Networking Practice Question
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.
⚠ Common exam trap
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.
Answer choices
Why each option matters
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
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.
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.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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