The root cause is that the EIGRP stub configuration on R1 prevents a full EIGRP adjacency, leaving it stuck in Init state, while the BFD session remains unaffected and fully operational. This occurs because BFD operates independently at the forwarding plane to detect link failures, so a BFD session can be Up even when the EIGRP neighbor relationship fails to complete. In this scenario, R1 is configured with the eigrp stub command but lacks the receive-only keyword, which means it does not advertise routes to R2, causing the EIGRP adjacency to stall in Init despite BFD being healthy. On the Cisco CCNP ENARSI 300-410 exam, this question tests your ability to separate BFD behavior from EIGRP adjacency mechanics, a common trap where candidates blame BFD for an EIGRP-specific issue. Remember: BFD is a transport-agnostic helper; a BFD Up does not guarantee EIGRP adjacency success. Memory tip: “BFD checks the road, EIGRP checks the cargo—stub without receive-only leaves the truck empty.”
300-410 Bidirectional Forwarding Detection (BFD) Practice Question
This 300-410 practice question tests your understanding of bidirectional forwarding detection (bfd). The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
R1 and R2 are EIGRP neighbors with BFD enabled. R1#show ip eigrp neighbors shows R2 in state 'Init' for BFD. R1#show bfd neighbors shows the session as 'Up'. R2#show bfd neighbors shows the session as 'Up'. R1 has 'eigrp stub' configured. R2 does not. 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 EIGRP stub configuration on R1 prevents full adjacency, but BFD is unaffected.
EIGRP stub routers can form BFD sessions but the EIGRP adjacency may remain in Init state if the stub router is not configured correctly. However, BFD session is up, so the issue is with EIGRP itself. The stub router may be blocking queries, but that does not affect BFD. The real issue is that R1 has 'eigrp stub' but is missing the 'receive-only' keyword, causing it to not advertise routes, but BFD is fine. The BFD session is up, so the problem is EIGRP, not BFD.
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 EIGRP stub configuration on R1 prevents full adjacency, but BFD is unaffected.
Why this is correct
EIGRP stub does not affect BFD; the BFD session is up, but EIGRP adjacency is in Init due to stub behavior.
EIGRP must be configured with 'no eigrp stub' for BFD to work.
Why it's wrong here
BFD works regardless of stub.
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 EIGRP stub configuration on R1 prevents full adjacency, but BFD is unaffected. — EIGRP stub routers can form BFD sessions but the EIGRP adjacency may remain in Init state if the stub router is not configured correctly. However, BFD session is up, so the issue is with EIGRP itself. The stub router may be blocking queries, but that does not affect BFD. The real issue is that R1 has 'eigrp stub' but is missing the 'receive-only' keyword, causing it to not advertise routes, but BFD is fine. The BFD session is up, so the problem is EIGRP, not BFD.
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.
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