This CISSP practice question tests your understanding of cissp exam topics. 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.
Refer to the exhibit. A VPN tunnel between two routers is not establishing. Which of the following 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.
The pre-shared key is configured for any peer, which is a security risk but not the cause of failure.
Why wrong: The key is configured for any peer (0.0.0.0), which works but is not best practice.
B
The crypto map is applied to the physical interface, but a tunnel interface is also configured for the same peer, causing a conflict.
Having both a crypto map on the physical interface and a tunnel interface to the same peer creates a conflict and prevents the tunnel from establishing.
C
The ISAKMP policy uses AES 256 encryption, but the transform set also uses AES 256, causing a mismatch.
Why wrong: Both are AES 256, so they are compatible.
D
The access list 100 does not match the traffic correctly; it should be a permit statement for the VPN traffic.
Why wrong: The access list matches traffic from 192.168.1.0/24 to 192.168.2.0/24, which is typical for VPN traffic.
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 physical interface, but a tunnel interface is also configured for the same peer, causing a conflict.
The crypto map is applied to the physical interface GigabitEthernet0/0, but the tunnel interface Tunnel0 is also configured with the same tunnel destination. The crypto map and the tunnel interface are both trying to establish a tunnel to the same peer, which causes a conflict. Typically, you would use either a crypto map or a tunnel interface, not both. The crypto map is applied to the physical interface, but the tunnel interface is also configured. This can prevent the tunnel from establishing.
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 pre-shared key is configured for any peer, which is a security risk but not the cause of failure.
Why it's wrong here
The key is configured for any peer (0.0.0.0), which works but is not best practice.
✓
The crypto map is applied to the physical interface, but a tunnel interface is also configured for the same peer, causing a conflict.
Why this is correct
Having both a crypto map on the physical interface and a tunnel interface to the same peer creates a conflict and prevents the tunnel from establishing.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
✗
The ISAKMP policy uses AES 256 encryption, but the transform set also uses AES 256, causing a mismatch.
Why it's wrong here
Both are AES 256, so they are compatible.
✗
The access list 100 does not match the traffic correctly; it should be a permit statement for the VPN traffic.
Why it's wrong here
The access list matches traffic from 192.168.1.0/24 to 192.168.2.0/24, which is typical for VPN traffic.
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 developer is choosing between AES-256 (symmetric) and RSA-2048 (asymmetric) for encrypting a large file that will be sent to a partner. Symmetric encryption is fast but requires key exchange; asymmetric is slower but solves the key distribution problem. A hybrid approach — encrypt the file with AES, encrypt the AES key with RSA — is standard. Questions like this test whether you understand when each approach applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CISSP question in full detail.
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 CISSP ACL questions on filtering logic and placement.
The correct answer is: The crypto map is applied to the physical interface, but a tunnel interface is also configured for the same peer, causing a conflict. — The crypto map is applied to the physical interface GigabitEthernet0/0, but the tunnel interface Tunnel0 is also configured with the same tunnel destination. The crypto map and the tunnel interface are both trying to establish a tunnel to the same peer, which causes a conflict. Typically, you would use either a crypto map or a tunnel interface, not both. The crypto map is applied to the physical interface, but the tunnel interface is also configured. This can prevent the tunnel from establishing.
What should I do if I get this CISSP 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 CISSP 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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Question Discussion
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