Question 1,329 of 2,152

IPsec Site-to-Site VPN →mediumMultiple ChoiceObjective-mapped

300-410 IPsec Site-to-Site VPN Practice Question

This 300-410 practice question tests your understanding of ipsec site-to-site vpn. 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.

A network engineer is troubleshooting an IPsec site-to-site VPN between two routers. The tunnel interface is up/up, but traffic from the local LAN to the remote LAN is not passing. The engineer checks the crypto map and sees it is applied to the outside interface. What is the most likely cause of the traffic failure?

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 1mediummultiple choice

Read the full VPN explanation →

A
The crypto map is not applied to the tunnel interface.
Why wrong: Incorrect because the crypto map should be applied to the physical outside interface, not the tunnel interface.
B
The access list in the crypto map does not match the LAN-to-LAN traffic.
Correct because the crypto map uses an access list to define which traffic is encrypted; if it does not match the actual LAN subnets, traffic will be sent unencrypted and may be dropped by the remote router.
C
The IPsec transform set is missing the esp-aes encryption algorithm.
Why wrong: Incorrect because the transform set configuration is for the encryption and authentication algorithms; a missing algorithm would cause phase 2 negotiation failure, but the tunnel is up.
D
The IKE phase 1 proposal is mismatched between the two routers.
Why wrong: Incorrect because if IKE phase 1 were mismatched, the tunnel would not come up at all.

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 access list in the crypto map does not match the LAN-to-LAN traffic.

The tunnel interface being up/up indicates the GRE tunnel is operational, but traffic may still fail if the crypto map is not correctly triggering IPsec encryption for the actual data traffic. The most common cause is a missing or incorrect access list in the crypto map that defines interesting 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 map is not applied to the tunnel interface.
Why it's wrong here
Incorrect because the crypto map should be applied to the physical outside interface, not the tunnel interface.
✓
The access list in the crypto map does not match the LAN-to-LAN traffic.
Why this is correct
Correct because the crypto map uses an access list to define which traffic is encrypted; if it does not match the actual LAN subnets, traffic will be sent unencrypted and may be dropped by the remote router.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
✗
The IPsec transform set is missing the esp-aes encryption algorithm.
Why it's wrong here
Incorrect because the transform set configuration is for the encryption and authentication algorithms; a missing algorithm would cause phase 2 negotiation failure, but the tunnel is up.
✗
The IKE phase 1 proposal is mismatched between the two routers.
Why it's wrong here
Incorrect because if IKE phase 1 were mismatched, the tunnel would not come up at all.

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 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. 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 exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.

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?

IPsec Site-to-Site VPN — This question tests IPsec Site-to-Site VPN — Standard ACLs match source addresses..

What is the correct answer to this question?

The correct answer is: The access list in the crypto map does not match the LAN-to-LAN traffic. — The tunnel interface being up/up indicates the GRE tunnel is operational, but traffic may still fail if the crypto map is not correctly triggering IPsec encryption for the actual data traffic. The most common cause is a missing or incorrect access list in the crypto map that defines interesting 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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