The answer is that the crypto map is not applied to the outside interface on Router B. For IPsec to function bidirectionally, the crypto map must be applied on both sides of the tunnel; if it is missing on one router, that router will not encrypt its outgoing traffic, even though it can decrypt incoming traffic from the peer. This scenario tests your understanding of the IKEv1 site-to-site VPN configuration process on the ENCOR 350-401 exam, where a common trap is assuming that a one-way tunnel means the access list or transform set is misconfigured, when in fact the crypto map is simply not attached to the correct interface on the remote router. A helpful memory tip is “both sides, both directions”—if the crypto map is not applied to the outside interface on both routers, you will only get one-way encryption.
CCNP VPN Technologies Practice Question
This 350-401 practice question tests your understanding of vpn technologies. 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.
An engineer is configuring a site-to-site VPN between two Cisco routers using IPsec with IKEv1. The engineer configures a crypto map on the outside interface. The tunnel establishes, but only traffic from one direction is encrypted. For example, traffic from Router A to Router B is encrypted, but traffic from Router B to Router A is not. The engineer checks the crypto map on Router B and finds that it is not applied to the correct interface. What is the most likely issue?
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 not applied to the outside interface on Router B.
For IPsec to work bidirectionally, the crypto map must be applied to the outside interface on both routers. If it is missing on one side, that router will not encrypt outgoing traffic. Option A is correct because the crypto map is not applied on Router B. Option B is incorrect because the access list is not the issue. Option C is incorrect because the transform set is used for encryption. Option D is incorrect because the peer is configured.
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 outside interface on Router B.
Why this is correct
Correct because the crypto map must be applied on both sides.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
✗
The access list on Router B is missing the permit statement for the traffic.
Why it's wrong here
If the access list were missing, no traffic would be encrypted.
✗
The transform set on Router B is different from Router A.
Why it's wrong here
Transform set mismatch would prevent tunnel establishment.
✗
The peer address on Router B is configured incorrectly.
Why it's wrong here
If the peer were wrong, 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.
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 350-401 ACL questions on filtering logic and placement.
VPN Technologies — This question tests VPN Technologies — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: The crypto map is not applied to the outside interface on Router B. — For IPsec to work bidirectionally, the crypto map must be applied to the outside interface on both routers. If it is missing on one side, that router will not encrypt outgoing traffic. Option A is correct because the crypto map is not applied on Router B. Option B is incorrect because the access list is not the issue. Option C is incorrect because the transform set is used for encryption. Option D is incorrect because the peer is configured.
What should I do if I get this 350-401 question wrong?
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 350-401 ACL questions on filtering logic and placement.
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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