The answer is that the crypto map is applied to the wrong interface. This is the most likely cause of traffic failure because a site-to-site VPN tunnel requires the crypto map to be applied to the interface that physically connects to the remote peer—typically the WAN-facing interface. When the map is mistakenly placed on an outbound interface like a loopback or LAN port, the router cannot match traffic to the correct encryption policy, so even though the ISAKMP and IPsec SAs are up, the router will not encrypt outbound traffic or decrypt inbound traffic for the VPN. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of crypto map placement as a fundamental VPN troubleshooting step; a common trap is assuming the tunnel being up guarantees traffic flow. Remember the memory tip: “Map the exit, not the entry”—the crypto map must always face the remote peer’s path.
300-410 Device Access Control Practice Question
This 300-410 practice question tests your understanding of device access control. 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 a site-to-site VPN between two Cisco routers. The tunnel is up, but traffic is not passing. On R1, the engineer issues the command 'show crypto map' and sees that the crypto map is applied to the outbound 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The crypto map is applied to the wrong interface.
The crypto map must be applied to the interface through which VPN traffic exits. If it is applied to the wrong interface (e.g., a loopback or a LAN interface instead of the WAN-facing interface), the router will not encrypt outbound traffic or decrypt inbound traffic for the VPN, even though the tunnel (ISAKMP/IPsec SA) may be established. The show crypto map output confirming the map is on the outbound interface indicates a misapplication, as the correct interface is the one facing the remote peer.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 applied to the wrong interface.
Why this is correct
Correct because crypto maps should be applied to the inbound direction of the interface to match traffic for encryption.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The access-list in the crypto map does not permit the traffic.
Why it's wrong here
Incorrect because the tunnel is up, indicating the IKE and IPsec negotiations succeeded; the issue is the crypto map application direction.
✗
The ISAKMP policy is misconfigured.
Why it's wrong here
Incorrect because the tunnel is up, so ISAKMP negotiation succeeded.
✗
The transform set is incorrect.
Why it's wrong here
Incorrect because the tunnel is up, meaning the transform set was accepted during negotiation.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that a tunnel being up guarantees traffic flow, but the trap here is that the crypto map must be applied to the correct interface (the one with the route to the remote peer) for encryption to occur, and candidates may overlook this by focusing on ACL or policy issues instead.
Detailed technical explanation
How to think about this question
Cisco IOS applies crypto maps to interfaces using the 'crypto map <name>' command under interface configuration mode. The crypto map defines which traffic to encrypt (via ACL) and which IPsec parameters to use. If applied to the wrong interface, the router will not match outgoing packets against the crypto map ACL, so no encryption occurs, and the IPsec SA remains idle. In real-world scenarios, this often happens when engineers apply the crypto map to a VLAN interface or a subinterface instead of the physical WAN interface, leading to a 'tunnel up, no traffic' symptom that is easily missed.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Device Access Control — This question tests Device Access Control — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The crypto map is applied to the wrong interface. — The crypto map must be applied to the interface through which VPN traffic exits. If it is applied to the wrong interface (e.g., a loopback or a LAN interface instead of the WAN-facing interface), the router will not encrypt outbound traffic or decrypt inbound traffic for the VPN, even though the tunnel (ISAKMP/IPsec SA) may be established. The show crypto map output confirming the map is on the outbound interface indicates a misapplication, as the correct interface is the one facing the remote peer.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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