Question 159 of 2,152

NetFlow and Flexible NetFlow →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the ACL on the remote router does not mirror the local ACL, so the remote router does not initiate an SA for the return traffic. This is because IPsec requires symmetric, mirrored ACLs on both peers to define interesting traffic in both directions; if the local router permits traffic from A to B, the remote must permit traffic from B to A, or the return path will lack a matching security association, leaving the traffic unencrypted despite the tunnel being up. On the Cisco CCNP ENARSI 300-410 exam, this tests your understanding of IPsec VPN ACL mirroring as a common edge case where engineers mistakenly configure one-way ACLs or apply the crypto map to the wrong interface, often confusing tunnel IPs with real IPs. A frequent trap is assuming a single ACL is sufficient, but the exam expects you to verify that both sides are exact opposites. Remember the memory tip: "Mirror, mirror—if one side permits A to B, the other must permit B to A, or your VPN traffic will go naked."

300-410 NetFlow and Flexible NetFlow Practice Question

This 300-410 practice question tests your understanding of netflow and flexible netflow. 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 IPsec between two routers using a site-to-site VPN. The tunnel is established, but traffic is not encrypted. The engineer checks the crypto map and sees that the ACL for interesting traffic is configured correctly. Which 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.

Question 1hardmultiple choice

Study the full ACL explanation →

A
The ACL on the remote router does not mirror the local ACL, so the remote router does not initiate an SA for the return traffic.
IPsec requires matching ACLs on both sides. If the remote ACL does not permit the return traffic, the SA will not be established for that direction, and traffic may be sent unencrypted.
B
The crypto map is applied to the wrong interface (e.g., the inside interface instead of the outside interface).
Why wrong: This would prevent encryption entirely, but the scenario says the tunnel is established, meaning the crypto map is likely applied correctly.
C
The transform set uses ESP with authentication only, which does not provide encryption.
Why wrong: ESP with authentication only (esp-md5-hmac) does not encrypt, but the scenario says the tunnel is established, and the engineer expects encryption. This could be a valid issue, but the question focuses on ACL mirroring.
D
The IKE policy uses aggressive mode, which does not support encryption.
Why wrong: Aggressive mode is a phase 1 negotiation method and does not affect encryption; it only affects identity protection.

Full breakdown with real-world context →

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 ACL on the remote router does not mirror the local ACL, so the remote router does not initiate an SA for the return traffic.

A common edge case is that the ACL for interesting traffic is applied to the wrong interface or in the wrong direction. In site-to-site VPNs, the crypto map is applied to the outbound interface of the traffic. However, if the ACL is configured with the wrong source/destination (e.g., using the tunnel IP instead of the real IP), traffic will not match. Another less obvious issue is that the ACL must be symmetric; if the ACL on one router permits traffic from A to B, the other router must permit traffic from B to A. If one side is missing, the traffic may be sent but not encrypted because the other side does not have a matching SA.

Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.

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 ACL on the remote router does not mirror the local ACL, so the remote router does not initiate an SA for the return traffic.
Why this is correct
IPsec requires matching ACLs on both sides. If the remote ACL does not permit the return traffic, the SA will not be established for that direction, and traffic may be sent unencrypted.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The crypto map is applied to the wrong interface (e.g., the inside interface instead of the outside interface).
Why it's wrong here
This would prevent encryption entirely, but the scenario says the tunnel is established, meaning the crypto map is likely applied correctly.
✗
The transform set uses ESP with authentication only, which does not provide encryption.
Why it's wrong here
ESP with authentication only (esp-md5-hmac) does not encrypt, but the scenario says the tunnel is established, and the engineer expects encryption. This could be a valid issue, but the question focuses on ACL mirroring.
✗
The IKE policy uses aggressive mode, which does not support encryption.
Why it's wrong here
Aggressive mode is a phase 1 negotiation method and does not affect encryption; it only affects identity protection.

Common exam traps

Common exam trap: NAT rules depend on direction and matching traffic

NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.

Trap categories for this question

Scenario analysis trap
This would prevent encryption entirely, but the scenario says the tunnel is established, meaning the crypto map is likely applied correctly.

Detailed technical explanation

How to think about this question

NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.

KKey Concepts to Remember

Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.

TExam Day Tips

Identify inside and outside interfaces first.
Check whether the scenario needs static NAT, dynamic NAT or PAT.
Do not confuse NAT matching ACLs with normal packet-filtering intent.

Key takeaway

NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.

Real-world example

How this comes up in practice

A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.

What to study next

Got this wrong? Here's your next step.

Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.

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FAQ

Questions learners often ask

What does this 300-410 question test?

NetFlow and Flexible NetFlow — This question tests NetFlow and Flexible NetFlow — Static NAT maps one inside address to one outside address..

What is the correct answer to this question?

The correct answer is: The ACL on the remote router does not mirror the local ACL, so the remote router does not initiate an SA for the return traffic. — A common edge case is that the ACL for interesting traffic is applied to the wrong interface or in the wrong direction. In site-to-site VPNs, the crypto map is applied to the outbound interface of the traffic. However, if the ACL is configured with the wrong source/destination (e.g., using the tunnel IP instead of the real IP), traffic will not match. Another less obvious issue is that the ACL must be symmetric; if the ACL on one router permits traffic from A to B, the other router must permit traffic from B to A. If one side is missing, the traffic may be sent but not encrypted because the other side does not have a matching SA.

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?

Static NAT maps one inside address to one outside address.

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