The answer is that the interface has no IP address configured. BFD for OSPF requires the interface to have an active IP address and be in an up/up state before a BFD session can be established; without an IP, the underlying Layer 3 adjacency cannot form, and BFD has no path to send or receive control packets. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that BFD configuration is not just about timers and enabling it under OSPF—the interface itself must be fully operational with an IP address. A common trap is focusing solely on the bfd all-interfaces command or the timer values while overlooking the basic prerequisite of IP connectivity. Remember the memory tip: “No IP, no BFD—check the interface first.”
300-410 Bidirectional Forwarding Detection (BFD) Practice Question
This 300-410 practice question tests your understanding of bidirectional forwarding detection (bfd). This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 has configured BFD for OSPF as shown:
interface GigabitEthernet0/0
bfd interval 300 min_rx 300 multiplier 3
!
router ospf 1
bfd all-interfaces
!
However, BFD sessions are not coming up. What is the most likely missing configuration?
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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The interface has no IP address configured.
BFD requires that the interface has an IP address and is up/up. The configuration shown lacks an IP address on the interface, which is necessary for BFD to establish a session.
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 'bfd all-interfaces' command should be replaced with 'bfd interface GigabitEthernet0/0'.
Why it's wrong here
'bfd all-interfaces' is valid and enables BFD on all interfaces; it is not the issue.
✓
The interface has no IP address configured.
Why this is correct
Correct. Without an IP address on the interface, OSPF cannot form a neighbor relationship, and BFD requires an established OSPF adjacency to operate.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The BFD timers must be set to 100 ms or less for OSPF.
Why it's wrong here
There is no such requirement; BFD timers can be any value supported by the platform.
✗
The router must be configured with 'bfd map' commands for each neighbor.
Why it's wrong here
BFD does not require 'bfd map' commands; that is used for static BFD, not for dynamic routing protocols.
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.
Trap categories for this question
Command / output trap
BFD does not require 'bfd map' commands; that is used for static BFD, not for dynamic routing protocols.
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 interface has no IP address configured. — BFD requires that the interface has an IP address and is up/up. The configuration shown lacks an IP address on the interface, which is necessary for BFD to establish a session.
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.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A network engineer runs the following command to troubleshoot BFD with OSPF:
R1# show ip ospf interface gigabitethernet 0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 10.1.1.1/24, Area 0
Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 1.1.1.1, Interface address 10.1.1.1
Backup Designated router (ID) 2.2.2.2, Interface address 10.1.1.2
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:03
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Index 1/1/1, flood queue length 0
Next 0x0(0)/0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 25
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 2.2.2.2 (Backup Designated Router)
Suppress hello for 0 neighbor(s)
BFD enabled, BFD interval 100 msec, BFD multiplier 3
What does this output indicate?
medium
✓ A.OSPF is using BFD with a 100 ms interval and multiplier 3, enabling sub-second failure detection.
B.BFD is disabled on this OSPF interface.
C.BFD interval is set to 10 seconds, matching OSPF hello.
D.BFD session is down, causing OSPF to use dead timer.
Why A: The output shows that BFD is enabled on the OSPF interface with an interval of 100 ms and multiplier of 3. This provides fast failure detection for OSPF adjacency.
Last reviewed: Jun 18, 2026
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