Question 260 of 2,152

Network Logging and Syslog →hardMultiple ChoiceObjective-mapped

IPsec VPN Traffic Not Encrypted — Crypto ACL Mirroring Issue

This 300-410 practice question tests your understanding of network logging and syslog. 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 an IPsec site-to-site VPN between two routers. The tunnel comes up, but no traffic is encrypted. The engineer verifies that the crypto map is applied to the outgoing interface and that the ACL defining interesting traffic is correct. 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 crypto ACL is not mirrored on both sides, so the SA is not established for the traffic
Correct. IPsec requires that the crypto ACLs be mirror images of each other for bidirectional traffic to be encrypted.
B
The transform-set is missing encryption or authentication
Why wrong: A missing transform-set would prevent the tunnel from coming up at all.
C
The crypto map is applied to the wrong interface
Why wrong: If the crypto map is applied to the wrong interface, the tunnel would not come up.
D
The IKE policy is not configured
Why wrong: Without IKE policy, the tunnel would not establish.

Full breakdown with real-world context →

Quick Answer

The answer is a crypto ACL mirroring mismatch, where the access lists defining interesting traffic are not identical on both peers. For IPsec to establish the correct security associations (SAs), the ACL on each router must be a mirror image of the other, meaning the source and destination networks are swapped. If one router’s ACL permits traffic from LAN A to LAN B, the peer’s ACL must permit traffic from LAN B to LAN A; if they are both configured with the same source and destination, the SA negotiation fails for that traffic, leaving the tunnel up but data unencrypted. On the Cisco CCNP ENARSI 300-410 exam, this is a classic trap—candidates often verify the crypto map and interface but overlook that the ACL direction is reversed, causing a silent failure. A reliable memory tip is “mirror, not match”: the ACLs must reflect each other’s source and destination, not be identical.

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 crypto ACL is not mirrored on both sides, so the SA is not established for the traffic

A common edge case is that the ACL defining interesting traffic is applied in the wrong direction. For IPsec, the ACL should match traffic that will be encrypted, and it must be applied to the crypto map. However, if the ACL is configured with the source and destination reversed (e.g., source is local network and destination is remote network on one router, but on the other router the ACL should also have source as local network), the traffic may not match. Additionally, if the ACL is not mirrored on both sides, the tunnel may come up but traffic will not be encrypted because the SA is not established for that traffic.

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 crypto ACL is not mirrored on both sides, so the SA is not established for the traffic
Why this is correct
Correct. IPsec requires that the crypto ACLs be mirror images of each other for bidirectional traffic to be encrypted.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
✗
The transform-set is missing encryption or authentication
Why it's wrong here
A missing transform-set would prevent the tunnel from coming up at all.
✗
The crypto map is applied to the wrong interface
Why it's wrong here
If the crypto map is applied to the wrong interface, the tunnel would not come up.
✗
The IKE policy is not configured
Why it's wrong here
Without IKE policy, the tunnel would not establish.

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.

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FAQ

Questions learners often ask

What does this 300-410 question test?

Network Logging and Syslog — This question tests Network Logging and Syslog — Standard ACLs match source addresses..

What is the correct answer to this question?

The correct answer is: The crypto ACL is not mirrored on both sides, so the SA is not established for the traffic — A common edge case is that the ACL defining interesting traffic is applied in the wrong direction. For IPsec, the ACL should match traffic that will be encrypted, and it must be applied to the crypto map. However, if the ACL is configured with the source and destination reversed (e.g., source is local network and destination is remote network on one router, but on the other router the ACL should also have source as local network), the traffic may not match. Additionally, if the ACL is not mirrored on both sides, the tunnel may come up but traffic will not be encrypted because the SA is not established for that traffic.

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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