Question 1,356 of 2,152

MPLS L3VPN →hardMultiple ChoiceObjective-mapped

IPsec Site-to-Site VPN: ACL Must Be Mirrored for Encryption

This 300-410 practice question tests your understanding of mpls l3vpn. 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.

An engineer configures IPsec between two PE routers in an MPLS L3VPN using a site-to-site VPN. The engineer configures a transform-set with ESP encryption (AES-256) and ESP authentication (SHA-256). The engineer also configures an ACL to define interesting traffic. The VPN tunnel comes up, but no traffic is encrypted. The engineer verifies that the ACL is correct and that the crypto map is applied to the interface. 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 ACL for interesting traffic is not mirrored on the remote router; it only permits traffic in one direction.
Correct. IPsec requires the ACL to be mirrored on both sides; otherwise, the traffic is not considered interesting for encryption.
B
The transform-set uses AES-256, which is not supported on the remote router.
Why wrong: Incorrect. If the transform-set were unsupported, the tunnel would not come up at all.
C
The 'crypto map' is applied to the wrong interface, such as the loopback instead of the physical interface.
Why wrong: Incorrect. If the crypto map were on the wrong interface, the tunnel might not come up, but the question states the tunnel is up.
D
The 'ipsec-over-tcp' command is missing, causing the IPsec packets to be dropped.
Why wrong: Incorrect. IPsec-over-TCP is optional and not required for standard IPsec operation.

Full breakdown with real-world context →

Quick Answer

The answer is that the ACL for interesting traffic is not mirrored on the remote router, permitting traffic in only one direction. This is the most likely explanation because IPsec requires the ACL on each peer to be a mirror image of the other—if the local ACL permits traffic from source to destination, the remote ACL must permit traffic from destination to source. Without this mirror, the tunnel can come up via IKE phase 1 and 2, but the router never classifies the traffic as interesting, so no encryption occurs even though the crypto map is correctly applied. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that a site-to-site VPN tunnel being up does not guarantee traffic encryption; the ACL mirror is a common trap where candidates assume a single-direction ACL is sufficient. A quick memory tip: "Tunnel up, no encrypt? Check the ACL mirror—one-way ACL means no crypto."

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 for interesting traffic is not mirrored on the remote router; it only permits traffic in one direction.

A common edge case with IPsec is that the ACL for interesting traffic must be mirrored on both sides. If the ACL on one side permits traffic from source to destination, the other side must permit traffic from destination to source. If the ACLs are not mirrored, the tunnel may come up (due to IKE) but no traffic will be encrypted because the router does not consider the traffic as interesting. Another edge case is that the crypto map is applied to the wrong interface or that the ACL is using the wrong protocol (e.g., TCP instead of IP). However, the most likely explanation here is that the ACL is not correctly matching the traffic in both directions.

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 for interesting traffic is not mirrored on the remote router; it only permits traffic in one direction.
Why this is correct
Correct. IPsec requires the ACL to be mirrored on both sides; otherwise, the traffic is not considered interesting for encryption.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
✗
The transform-set uses AES-256, which is not supported on the remote router.
Why it's wrong here
Incorrect. If the transform-set were unsupported, the tunnel would not come up at all.
✗
The 'crypto map' is applied to the wrong interface, such as the loopback instead of the physical interface.
Why it's wrong here
Incorrect. If the crypto map were on the wrong interface, the tunnel might not come up, but the question states the tunnel is up.
✗
The 'ipsec-over-tcp' command is missing, causing the IPsec packets to be dropped.
Why it's wrong here
Incorrect. IPsec-over-TCP is optional and not required for standard IPsec operation.

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.

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.

Related 300-410 practice-question pages

Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.

Layer 3 Technologies practice questions

Practise 300-410 questions linked to Layer 3 Technologies.

EIGRP Troubleshooting practice questions

Practise 300-410 questions linked to EIGRP Troubleshooting.

OSPF Troubleshooting (v2/v3) practice questions

Practise 300-410 questions linked to OSPF Troubleshooting (v2/v3).

BGP Troubleshooting practice questions

Practise 300-410 questions linked to BGP Troubleshooting.

Route Redistribution practice questions

Practise 300-410 questions linked to Route Redistribution.

Policy-Based Routing (PBR) practice questions

Practise 300-410 questions linked to Policy-Based Routing (PBR).

VRF-Lite practice questions

Practise 300-410 questions linked to VRF-Lite.

Route Maps and Route Filtering practice questions

Practise 300-410 questions linked to Route Maps and Route Filtering.

Administrative Distance practice questions

Practise 300-410 questions linked to Administrative Distance.

Route Summarization practice questions

Practise 300-410 questions linked to Route Summarization.

Bidirectional Forwarding Detection (BFD) practice questions

Practise 300-410 questions linked to Bidirectional Forwarding Detection (BFD).

VPN Technologies practice questions

Practise 300-410 questions linked to VPN Technologies.

Practice this exam

Start a free 300-410 practice session

Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.

10 questions 20 questions 30 questions 50 questions Timed 30

300-410 practice-test guide →Study guide →Browse all practice tests

FAQ

Questions learners often ask

What does this 300-410 question test?

MPLS L3VPN — This question tests MPLS L3VPN — Standard ACLs match source addresses..

What is the correct answer to this question?

The correct answer is: The ACL for interesting traffic is not mirrored on the remote router; it only permits traffic in one direction. — A common edge case with IPsec is that the ACL for interesting traffic must be mirrored on both sides. If the ACL on one side permits traffic from source to destination, the other side must permit traffic from destination to source. If the ACLs are not mirrored, the tunnel may come up (due to IKE) but no traffic will be encrypted because the router does not consider the traffic as interesting. Another edge case is that the crypto map is applied to the wrong interface or that the ACL is using the wrong protocol (e.g., TCP instead of IP). However, the most likely explanation here is that the ACL is not correctly matching the traffic in both directions.

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.

About these practice questions

Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →

How Courseiva writes practice questions · Editorial policy

Keep practising

Question Discussion

Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.

Loading comments…

This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.