Question 1,198 of 2,152

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

Quick Answer

The answer is a misconfigured route on the remote router that sends return traffic out the wrong interface. When the IPsec VPN tunnel is up but you see a packet encrypt/decrypt mismatch—where encrypted packets far outnumber decrypted ones on the remote side—it indicates that the remote router is encrypting its outbound traffic but failing to decrypt the inbound traffic because those packets never reach the tunnel. This happens because the remote router’s routing table directs return traffic out a physical interface instead of the virtual tunnel interface, causing the local router to encrypt packets that the remote side never processes. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that a tunnel being up does not guarantee traffic is flowing correctly; the common trap is to blame encryption algorithms or security associations. Remember the memory tip: “If the tunnel is up but the counts don’t match, check the route—not the crypto.”

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 where the tunnel is up, but the engineer notices that the 'show crypto ipsec sa' output shows that the number of packets encrypted is much higher than the number of packets decrypted on the remote side. What is the most likely cause?

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

Read the full VPN explanation →

A
The remote router has a misconfigured route that sends return traffic out the wrong interface.
Correct because if the remote router does not have a route to the local LAN that points to the tunnel interface, the return traffic will be sent out the physical interface without encryption, and the local router will not see corresponding decrypted packets.
B
The IPsec SA lifetime is set too low, causing frequent rekeying.
Why wrong: Incorrect because frequent rekeying would cause brief interruptions but not a persistent mismatch in packet counts.
C
The crypto map on the local router is applied to the wrong interface.
Why wrong: Incorrect because if the crypto map were on the wrong interface, the tunnel would likely not be up.
D
The access list in the crypto map on the remote router is too permissive, encrypting extra traffic.
Why wrong: Incorrect because that would cause the remote router to encrypt more packets, not fewer.

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 remote router has a misconfigured route that sends return traffic out the wrong interface.

A significant mismatch between encrypted and decrypted packet counts suggests that some packets are being lost or dropped after encryption. The most common cause is a routing issue where the return traffic from the remote side is not taking the VPN tunnel, so the remote router does not decrypt those packets.

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 remote router has a misconfigured route that sends return traffic out the wrong interface.
Why this is correct
Correct because if the remote router does not have a route to the local LAN that points to the tunnel interface, the return traffic will be sent out the physical interface without encryption, and the local router will not see corresponding decrypted packets.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
✗
The IPsec SA lifetime is set too low, causing frequent rekeying.
Why it's wrong here
Incorrect because frequent rekeying would cause brief interruptions but not a persistent mismatch in packet counts.
✗
The crypto map on the local router is applied to the wrong interface.
Why it's wrong here
Incorrect because if the crypto map were on the wrong interface, the tunnel would likely not be up.
✗
The access list in the crypto map on the remote router is too permissive, encrypting extra traffic.
Why it's wrong here
Incorrect because that would cause the remote router to encrypt more packets, not fewer.

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 remote router has a misconfigured route that sends return traffic out the wrong interface. — A significant mismatch between encrypted and decrypted packet counts suggests that some packets are being lost or dropped after encryption. The most common cause is a routing issue where the return traffic from the remote side is not taking the VPN tunnel, so the remote router does not decrypt those packets.

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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