hardMultiple ChoiceObjective-mapped
300-410 Practice Question: R1 and R2 are IS-IS neighbors with BFD enabled
R1 and R2 are IS-IS neighbors with BFD enabled. R1#show clns is-neighbors shows R2 as 'Up'. R1#show bfd neighbors shows the session as 'Down'. R2#show bfd neighbors shows the session as 'Up' with R1. R1 has 'bfd interval 50 min_rx 50 multiplier 3' on the interface. R2 has 'bfd interval 100 min_rx 100 multiplier 3'. The link is stable. What is the root cause?
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
✓
R1 has an ACL blocking UDP port 3784 from R2.
IS-IS BFD requires that the BFD session be established between the same IP addresses used for IS-IS. If R1's BFD session is down but R2's is up, it indicates a unidirectional issue. This can be due to an MTU mismatch where R1's BFD packets are fragmented and dropped, or a firewall blocking inbound BFD packets on R1. Here, the most likely cause is that R1 has an ACL denying UDP port 3784 from R2.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
R1 has an ACL blocking UDP port 3784 from R2.
Why this is correct
BFD uses UDP port 3784; if R1 blocks incoming BFD packets, the session appears down on R1 but up on R2 because R2 receives R1's packets.
- ✗
IS-IS requires 'bfd all-interfaces' to work with BFD.
Why it's wrong here
Per-interface BFD is supported.
- ✗
The BFD multiplier on R1 is too low.
Why it's wrong here
Multiplier 3 is standard.
- ✗
The IS-IS metric must be set to 1 for BFD.
Why it's wrong here
Metric does not affect BFD.
Visual reference
Go deeper
Related to this question
About these practice questions
One of 1,966 original 300-410 practice questions on Courseiva, each with a full explanation and wrong-answer analysis — not exam dumps or protected exam content. Learn why practice questions differ from exam dumps →
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 practice question is part of Courseiva's free Cisco 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 300-410 exam.