Question 159 of 2,152

NetFlow and Flexible NetFlow →hardMultiple ChoiceObjective-mapped

IPsec VPN ACL Mirroring Issue

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.

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 →

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."

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: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.

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
Standard ACLs match source addresses.
✗
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: ACLs stop at the first match

ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.

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

ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.

KKey Concepts to Remember

Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.

TExam Day Tips

Check inbound versus outbound direction.
Read the ACL from top to bottom.
Look for a broader permit or deny above the intended line.

Key takeaway

ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.

Real-world example

How this comes up in practice

A security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.

Visual reference

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.

Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related 300-410 ACL questions on filtering logic and placement.

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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 — Standard ACLs match source addresses..

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?

Standard ACLs match source addresses.

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