The answer is that R1 is missing the 'bfd interval' configuration on the interface facing R2. This is correct because BFD for BGP fall-over requires bidirectional BFD session establishment before BGP can transition out of the 'Active' state; if R2 sees the session as 'Up' but R1 sees it as 'Down', it indicates a unidirectional failure where R1 is not sending or processing BFD control packets. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that BFD must be enabled at both the BGP neighbor level (with 'fall-over bfd') and the interface level (with 'bfd interval'), and a common trap is assuming the BGP configuration alone is sufficient. Remember that BFD is a per-interface protocol—without the 'bfd interval' command on the interface, the router will not initiate the BFD session, leaving it stuck in a down state even when the link is stable. Memory tip: "BGP fall-over needs BFD to talk—if the interface is silent, the session will balk."
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 BGP peers with BFD enabled. R1#show bgp neighbors 10.1.1.2 shows BGP state 'Active' and BFD session 'Down'. R2#show bfd neighbors shows the session as 'Up' with R1. R1 has 'neighbor 10.1.1.2 fall-over bfd' configured. R2 has 'neighbor 10.1.1.1 fall-over bfd' configured. The link between them is stable. 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
✓
R1 is missing 'bfd interval' configuration on the interface facing R2.
BFD for BGP fall-over requires that the BFD session be established before BGP can form. If the BFD session is up on R2 but down on R1, it indicates a unidirectional issue. This can happen if R1 has an ACL blocking BFD control packets from R2, or if R1's BFD configuration is missing. Here, R1 likely lacks 'bfd interval' configuration on the interface, causing BFD to not initiate.
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.
✓
R1 is missing 'bfd interval' configuration on the interface facing R2.
Why this is correct
Without BFD interval configuration on the interface, R1 cannot establish a BFD session, causing BGP fall-over to keep BGP in Active state.
BGP requires 'neighbor 10.1.1.2 ebgp-multihop' for BFD to work.
Why it's wrong here
Multihop is not needed for directly connected peers.
✗
R2 has a higher BFD multiplier causing session failure.
Why it's wrong here
BFD session is up on R2, so multiplier is not an issue.
✗
The BGP update-source is not set to the interface IP.
Why it's wrong here
Update-source does not 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: R1 is missing 'bfd interval' configuration on the interface facing R2. — BFD for BGP fall-over requires that the BFD session be established before BGP can form. If the BFD session is up on R2 but down on R1, it indicates a unidirectional issue. This can happen if R1 has an ACL blocking BFD control packets from R2, or if R1's BFD configuration is missing. Here, R1 likely lacks 'bfd interval' configuration on the interface, causing BFD to not initiate.
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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