JNCIS-ENT IS IS Practice Question
An engineer is troubleshooting IS-IS adjacency formation failures on a Junos device. Which THREE parameters must match or be compatible between neighboring routers to successfully establish an IS-IS adjacency? (Choose three)
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
✓
Link-level MTU size, as oversized IIH packets may be dropped if they exceed the interface MTU.
IS-IS adjacencies require matching area addresses (for Level 1), compatible link/network types, and correct authentication settings if configured. System IDs must be unique, and maximum transmission unit (MTU) mismatches can prevent IIH packet processing.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Link-level MTU size, as oversized IIH packets may be dropped if they exceed the interface MTU.
Why this is correct
If the MTU is mismatched and padding causes IIH packets to exceed the smaller MTU, adjacency formation will fail.
- ✓
Area addresses (at least one shared area address for Level 1 adjacencies)
Why this is correct
For Level 1 adjacencies to form, routers must share at least one common area address.
- ✗
Identical system IDs configured on both neighboring nodes
Why it's wrong here
System IDs must be unique across the entire IS-IS routing domain; identical system IDs cause conflicts.
- ✓
Authentication type and password (if authentication is enabled)
Why this is correct
Mismatched authentication passwords or types will cause IS-IS hello packets to be rejected, preventing adjacency formation.
- ✗
Identical DIS priority values on multi-access networks
Why it's wrong here
DIS priorities do not need to be identical; the highest priority wins the election regardless.
About these practice questions
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed August 2026 · checked against the official Juniper Networks exam blueprint
This JNCIS-ENT practice question is part of Courseiva's free Juniper Networks certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the JNCIS-ENT exam.