Question 1,163 of 2,152

Bidirectional Forwarding Detection (BFD) →hardMultiple ChoiceObjective-mapped

BFD DMVPN Phase 2 Spoke-to-Spoke IPsec Failure

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.

An engineer configures BFD on a DMVPN Phase 2 spoke-to-spoke tunnel. The BFD session between two spokes comes up, but the spoke-to-spoke dynamic IPsec tunnel fails to establish. What is the most likely explanation?

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.

A
The BFD session is using the mGRE tunnel interface, but the IPsec crypto map is applied to the physical interface, causing a mismatch in the path.
Correct. In DMVPN, BFD should be configured on the tunnel interface, but IPsec is applied to the physical interface. If BFD is misconfigured to use the physical interface, it will not detect failures of the tunnel path, and the IPsec tunnel may not be triggered correctly.
B
The spoke routers have different BFD minimum intervals, causing the session to flap and reset the IPsec tunnel.
Why wrong: Incorrect. BFD interval mismatch can cause flapping, but it would affect the BFD session itself, not prevent IPsec establishment.
C
The NHRP authentication string is mismatched between spokes, preventing NHRP from resolving the destination.
Why wrong: Incorrect. NHRP authentication mismatch would prevent NHRP registration, but the BFD session would not come up at all if NHRP is not working.
D
The IPsec transform-set uses ESP with SHA-1, but BFD requires MD5 authentication.
Why wrong: Incorrect. BFD and IPsec are independent; BFD does not require any specific IPsec settings.

Full breakdown with real-world context →

Quick Answer

The answer is a mismatch between the BFD session path and the IPsec crypto map application. BFD on a DMVPN Phase 2 spoke-to-spoke tunnel runs over the mGRE tunnel interface, which handles the routing and NHRP redirects, while the IPsec crypto map is typically applied to the physical interface for encryption. This creates a path asymmetry where BFD can falsely indicate a healthy link, but the dynamic IPsec tunnel fails to establish because the crypto engine expects traffic to traverse the physical interface with matching security associations. On the Cisco CCNP ENARSI 300-410 exam, this tests your understanding of DMVPN Phase 2 architecture and the critical distinction between tunnel-level and physical-level encryption policies—a common trap is assuming BFD and IPsec share the same interface. Remember: BFD loves the tunnel, but IPsec lives on the wire; if they don’t align, the tunnel won’t fire.

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 BFD session is using the mGRE tunnel interface, but the IPsec crypto map is applied to the physical interface, causing a mismatch in the path.

In DMVPN Phase 2, dynamic spoke-to-spoke tunnels require IPsec encryption. BFD is configured on the mGRE tunnel interface to monitor connectivity. However, if the IPsec crypto map is applied to the physical interface (e.g., GigabitEthernet0/0) rather than the mGRE tunnel interface, there is a mismatch: BFD operates over the tunnel interface, while IPsec encryption is attempted on the physical interface. This prevents the IPsec SA from being established correctly for spoke-to-spoke traffic, even though the BFD session may come up over the tunnel.

Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

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 session is using the mGRE tunnel interface, but the IPsec crypto map is applied to the physical interface, causing a mismatch in the path.
Why this is correct
Correct. In DMVPN, BFD should be configured on the tunnel interface, but IPsec is applied to the physical interface. If BFD is misconfigured to use the physical interface, it will not detect failures of the tunnel path, and the IPsec tunnel may not be triggered correctly.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The spoke routers have different BFD minimum intervals, causing the session to flap and reset the IPsec tunnel.
Why it's wrong here
Incorrect. BFD interval mismatch can cause flapping, but it would affect the BFD session itself, not prevent IPsec establishment.
✗
The NHRP authentication string is mismatched between spokes, preventing NHRP from resolving the destination.
Why it's wrong here
Incorrect. NHRP authentication mismatch would prevent NHRP registration, but the BFD session would not come up at all if NHRP is not working.
✗
The IPsec transform-set uses ESP with SHA-1, but BFD requires MD5 authentication.
Why it's wrong here
Incorrect. BFD and IPsec are independent; BFD does not require any specific IPsec settings.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.

Detailed technical explanation

How to think about this question

This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.

KKey Concepts to Remember

Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
Use explanations to understand the rule behind the answer.

TExam Day Tips

Underline the problem statement mentally.
Watch for words such as best, first, most likely and least administrative effort.
Review why wrong options are wrong, not only why the correct option is correct.

Key takeaway

Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

Real-world example

How this comes up in practice

A practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.

Quick reference

VPN Protocol Comparison

Protocol	Port	Encryption	Authentication	Use Case
IKEv2 / IPsec	UDP 500 / 4500	AES-256	Certificates / PSK	Site-to-site & remote access
SSL / TLS VPN	TCP 443	TLS 1.3	Certificates / MFA	Clientless remote access
L2TP / IPsec	UDP 1701	AES (IPsec)	PSK / Certificates	Legacy remote access
WireGuard	UDP 51820	ChaCha20	Public keys	Modern high-performance VPN
PPTP	TCP 1723	MPPE (weak)	MS-CHAPv2	Legacy — avoid in production

PPTP is considered insecure. IKEv2/IPsec and SSL VPN are the current recommended options.

What to study next

Got this wrong? Here's your next step.

Identify which 300-410 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.

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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) — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The BFD session is using the mGRE tunnel interface, but the IPsec crypto map is applied to the physical interface, causing a mismatch in the path. — In DMVPN Phase 2, dynamic spoke-to-spoke tunnels require IPsec encryption. BFD is configured on the mGRE tunnel interface to monitor connectivity. However, if the IPsec crypto map is applied to the physical interface (e.g., GigabitEthernet0/0) rather than the mGRE tunnel interface, there is a mismatch: BFD operates over the tunnel interface, while IPsec encryption is attempted on the physical interface. This prevents the IPsec SA from being established correctly for spoke-to-spoke traffic, even though the BFD session may come up over the tunnel.

What should I do if I get this 300-410 question wrong?

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?

Read the scenario before looking for a memorised answer.

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