- A
Verify that the 'bfd' command is configured under the OSPF process.
For OSPF BFD to work, the 'bfd' command must be enabled under the OSPF routing process (router ospf X, then 'bfd all-interfaces' or per-neighbor). Without it, the BFD session will not be initiated.
- B
Check if the serial interface is configured with 'encapsulation ppp'.
Why wrong: Encapsulation type does not affect BFD; BFD works over HDLC, PPP, and Frame Relay.
- C
Ensure that the 'ip ospf bfd' command is also configured on the neighbor router.
Why wrong: While both sides need BFD, the absence on one side would cause the BFD session to be down, but the first step is to check the routing process configuration.
- D
Verify that the serial interface clock rate is set correctly.
Why wrong: Clock rate affects physical layer, but if the interface is up/up, clocking is likely correct.
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 an OSPF adjacency that fails to come up. Both routers are directly connected via a serial link. BFD is enabled on the interface. The engineer sees that the BFD session is down. The OSPF configuration shows 'ip ospf bfd' under the interface. The serial interface is up/up. What should the engineer check first?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"first"Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
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
Verify that the 'bfd' command is configured under the OSPF process.
BFD on serial interfaces often requires the 'bfd interval' command to be explicitly configured, as the default may be disabled. Also, serial interfaces may need the 'bfd' command under the routing protocol as well.
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.
- ✓
Verify that the 'bfd' command is configured under the OSPF process.
Why this is correct
For OSPF BFD to work, the 'bfd' command must be enabled under the OSPF routing process (router ospf X, then 'bfd all-interfaces' or per-neighbor). Without it, the BFD session will not be initiated.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
Check if the serial interface is configured with 'encapsulation ppp'.
Why it's wrong here
Encapsulation type does not affect BFD; BFD works over HDLC, PPP, and Frame Relay.
- ✗
Ensure that the 'ip ospf bfd' command is also configured on the neighbor router.
Why it's wrong here
While both sides need BFD, the absence on one side would cause the BFD session to be down, but the first step is to check the routing process configuration.
- ✗
Verify that the serial interface clock rate is set correctly.
Why it's wrong here
Clock rate affects physical layer, but if the interface is up/up, clocking is likely correct.
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: Verify that the 'bfd' command is configured under the OSPF process. — BFD on serial interfaces often requires the 'bfd interval' command to be explicitly configured, as the default may be disabled. Also, serial interfaces may need the 'bfd' command under the routing protocol as well.
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: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
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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