The answer is an IPsec transform set mismatch between the two routers. When an IKE SA is up but the IPsec SA is missing, phase 1 authentication and key exchange succeeded, but phase 2 negotiation failed because the peers cannot agree on the encryption, hash, or mode parameters defined in the transform set. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your ability to isolate phase 2 failures from phase 1 issues, a common trap where engineers focus on pre-shared keys or ACLs when the real culprit is a mismatched transform set or crypto map access list. Remember that an active IKE SA only guarantees the tunnel management channel is alive, not that the data-plane security associations are established. A quick memory tip: “IKE up, IPsec down? Check the transform set crown.”
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 that stopped working after a recent configuration change. The engineer runs 'show crypto isakmp sa' and sees an active IKE SA, but 'show crypto ipsec sa' shows no IPsec SAs. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The IPsec transform set on one router does not match the transform set on the other.
An active IKE SA indicates that IKE phase 1 completed successfully, but no IPsec SAs means phase 2 failed. The most common cause is a mismatch in the IPsec transform set or the crypto map access list between the two routers.
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 IPsec transform set on one router does not match the transform set on the other.
Why this is correct
Correct because during phase 2, the routers negotiate the transform set; if they do not match, the IPsec SA cannot be established.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
✗
The pre-shared key is incorrect on one of the routers.
Why it's wrong here
Incorrect because a pre-shared key mismatch would cause IKE phase 1 to fail, not phase 2.
✗
The tunnel interface is down.
Why it's wrong here
Incorrect because the issue is with IPsec SAs, not the tunnel interface.
✗
The IKE proposal is mismatched.
Why it's wrong here
Incorrect because an IKE proposal mismatch would cause phase 1 to fail, but phase 1 is active.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 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 300-410 ACL questions on filtering logic and placement.
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 IPsec transform set on one router does not match the transform set on the other. — An active IKE SA indicates that IKE phase 1 completed successfully, but no IPsec SAs means phase 2 failed. The most common cause is a mismatch in the IPsec transform set or the crypto map access list between the two routers.
What should I do if I get this 300-410 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 300-410 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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