The root cause is an OSPF network type mismatch between R1 and R2, which directly prevents BFD session establishment. BFD relies on consistent interface parameters—including the OSPF network type—to negotiate and maintain a session; when R1 is set to point-to-point while R2 remains on the default broadcast mode for FastEthernet, the two routers disagree on how the link should be treated, causing BFD to fail even though OSPF adjacency itself may form. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that BFD is tightly coupled with OSPF interface properties, and a common trap is assuming OSPF neighbor formation guarantees BFD success. A useful memory tip: “BFD mirrors OSPF’s network type—if they don’t match, BFD won’t latch.”
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.
Router R1 and R2 are OSPF neighbors over a FastEthernet link with BFD enabled. R1 has 'ip ospf network point-to-point' configured. R2 does not. After a reload, BFD sessions fail to establish. R1#show bfd neighbors shows no sessions. R2#show bfd neighbors shows no sessions. 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
✓
OSPF network type mismatch between R1 and R2 prevents BFD session establishment.
BFD requires matching OSPF network types on the same link for proper session establishment. When R1 is configured as point-to-point and R2 remains as broadcast, OSPF forms a neighbor relationship but BFD sessions fail because BFD expects consistent interface parameters.
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.
✗
BFD is not supported on FastEthernet interfaces.
Why it's wrong here
BFD is supported on FastEthernet interfaces.
✓
OSPF network type mismatch between R1 and R2 prevents BFD session establishment.
Why this is correct
Mismatched OSPF network types cause BFD to fail because BFD uses interface parameters derived from OSPF network type.
BFD timers can be default; global configuration is not mandatory.
✗
OSPF hello/dead intervals must match for BFD to work.
Why it's wrong here
OSPF hello/dead intervals do not directly affect BFD session establishment.
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: OSPF network type mismatch between R1 and R2 prevents BFD session establishment. — BFD requires matching OSPF network types on the same link for proper session establishment. When R1 is configured as point-to-point and R2 remains as broadcast, OSPF forms a neighbor relationship but BFD sessions fail because BFD expects consistent interface parameters.
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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