The answer is a mismatch between the LDP router-id and the interface IP where BFD is enabled. BFD for LDP requires that the BFD session be established over the same interface as the LDP session; if the LDP router-id is set to a loopback address rather than the physical interface IP used for BFD, the BFD session will fail even though the LDP neighbor relationship appears configured. This scenario directly tests your understanding of the BFD integration with LDP on the Cisco CCNP ENARSI 300-410 exam, where a common trap is assuming that matching router-ids alone guarantee BFD success. The key is that BFD tracks the source IP of the LDP hello packets, so the router-id must be the IP of the interface sending those hellos. Memory tip: BFD follows the hello, not the loopback.
300-410 Bidirectional Forwarding Detection (BFD) Practice Question
This 300-410 practice question tests your understanding of bidirectional forwarding detection (bfd). Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 an MPLS network, routers R1 and R2 are LDP neighbors with BFD enabled. R1#show mpls ldp neighbor shows R2 as 'Oper Down' for the BFD session. R2#show bfd neighbors shows the session with R1 as 'Down'. R1 and R2 have matching 'mpls ldp router-id' loopback0. The link between them is up. What is the root cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The LDP router-id must be the IP address of the interface where BFD is enabled.
BFD for LDP requires that the LDP session be established over the same interface as the BFD session. If the LDP router-id is not the interface IP used for BFD, the BFD session may fail. Here, the loopback0 is used as LDP router-id, but BFD is configured on the physical link, causing mismatch.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
The LDP router-id must be the IP address of the interface where BFD is enabled.
Why this is correct
BFD for LDP requires the LDP session to use the same source IP as the BFD session; otherwise, BFD cannot monitor the LDP session.
BFD interval does not cause session down state; it affects detection time.
✗
LDP must be configured with 'no bfd' to work properly.
Why it's wrong here
Removing BFD would not solve the root cause.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
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
How to think about this question
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.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
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
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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 300-410 OSPF questions on adjacency and route selection.
Bidirectional Forwarding Detection (BFD) — This question tests Bidirectional Forwarding Detection (BFD) — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The LDP router-id must be the IP address of the interface where BFD is enabled. — BFD for LDP requires that the LDP session be established over the same interface as the BFD session. If the LDP router-id is not the interface IP used for BFD, the BFD session may fail. Here, the loopback0 is used as LDP router-id, but BFD is configured on the physical link, causing mismatch.
What should I do if I get this 300-410 question wrong?
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 300-410 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
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.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.