- A
The BGP neighbor is not directly connected; BFD requires a directly connected interface or a static route pointing to the neighbor's IP.
BFD sessions over multihop BGP require special configuration (bfd all-interfaces under BGP) and a route to the neighbor; if the neighbor is not directly connected, BFD will fail without proper setup.
- B
The Layer 3 switch is not configured for BFD, causing it to drop BFD packets.
Why wrong: The switch does not need to run BFD; it only forwards packets. BFD packets are UDP and should be forwarded normally.
- C
The BGP session is using EBGP multihop, and the TTL is set to 1.
Why wrong: If TTL is 1, BGP packets would not reach the neighbor, but BFD packets would still be sent; the BFD session would still attempt to form.
- D
The interface is configured with 'bfd interval 50 min_rx 50 multiplier 3' but the neighbor is not configured for BFD.
Why wrong: If the neighbor is not configured for BFD, the BFD session will not form, but the engineer would see the session as 'Down' with no configuration on the neighbor.
Quick Answer
The answer is that the BFD session fails because the BGP neighbor is not directly connected. BFD for BGP requires the destination IP address to be reachable via a directly connected interface or a static route; when the neighbor is reached through a Layer 3 switch or a loopback interface, the next hop is not directly connected, causing BFD to remain in a 'Down' state even though the physical interface is up/up. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of BFD operational requirements versus BGP peering rules—a common trap is assuming BFD works automatically with any BGP session, but it specifically demands direct Layer 2 adjacency or a static route to the neighbor’s IP. Remember the memory tip: "BFD needs a direct feed"—if the neighbor isn't one hop away, BFD won't play.
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.
A network engineer is troubleshooting a BGP session that is not establishing. The routers are connected via a Layer 3 switch. BFD is configured for BGP. The engineer checks the BFD session and sees it is 'Down'. The BGP configuration appears correct. The interface between the routers is up/up. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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
The BGP neighbor is not directly connected; BFD requires a directly connected interface or a static route pointing to the neighbor's IP.
BFD sessions require that the destination IP address be reachable via a directly connected interface or a static route. If the BGP neighbor is not directly connected (e.g., via a loopback), BFD may fail if the next hop is not directly connected or if there is a routing issue.
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 BGP neighbor is not directly connected; BFD requires a directly connected interface or a static route pointing to the neighbor's IP.
Why this is correct
BFD sessions over multihop BGP require special configuration (bfd all-interfaces under BGP) and a route to the neighbor; if the neighbor is not directly connected, BFD will fail without proper setup.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The Layer 3 switch is not configured for BFD, causing it to drop BFD packets.
Why it's wrong here
The switch does not need to run BFD; it only forwards packets. BFD packets are UDP and should be forwarded normally.
- ✗
The BGP session is using EBGP multihop, and the TTL is set to 1.
Why it's wrong here
If TTL is 1, BGP packets would not reach the neighbor, but BFD packets would still be sent; the BFD session would still attempt to form.
- ✗
The interface is configured with 'bfd interval 50 min_rx 50 multiplier 3' but the neighbor is not configured for BFD.
Why it's wrong here
If the neighbor is not configured for BFD, the BFD session will not form, but the engineer would see the session as 'Down' with no configuration on the neighbor.
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.
What to study next
Got this wrong? Here's your next step.
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.
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Bidirectional Forwarding Detection (BFD) — study guide chapter
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FAQ
Questions learners often ask
What does this 300-410 question test?
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 BGP neighbor is not directly connected; BFD requires a directly connected interface or a static route pointing to the neighbor's IP. — BFD sessions require that the destination IP address be reachable via a directly connected interface or a static route. If the BGP neighbor is not directly connected (e.g., via a loopback), BFD may fail if the next hop is not directly connected or if there is a routing issue.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 18, 2026
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