The answer is that the debug crypto ipsec output indicates a transform set mismatch between the two IPsec peers, causing the IPsec SA negotiation to fail. This is confirmed by the message "transform proposal (esp-3des esp-sha-hmac) not supported for proxy," which means the remote peer either does not support or is configured with a different encryption or hash algorithm than what the local router proposed. In the Cisco CCNP ENARSI 300-410 exam, this scenario tests your ability to interpret debug crypto ipsec messages and identify why an IPsec tunnel fails to establish—a common trap is assuming the problem lies with ACLs or IKE phase 1 when the real issue is a mismatched transform set. The placeholder SPIs (0x1234, 0x5678) with no actual inbound or outbound SPI are a dead giveaway that negotiation never completed. Memory tip: when you see "not supported for proxy" in the debug output, think "transform mismatch" first—check both peers for identical encryption, hash, and mode settings.
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 runs the following command to troubleshoot an IPsec Site-to-Site VPN issue:
R1# debug crypto ipsec
*Mar 1 00:02:34.567: IPSEC(sa_request): , (key eng. msg.) src=10.0.0.1, dst=10.0.0.2, src_proxy=192.168.1.0/255.255.255.0/0/0, dst_proxy=192.168.2.0/255.255.255.0/0/0,
*Mar 1 00:02:34.567: IPSEC(validate_proposal): transform proposal (esp-3des esp-sha-hmac) not supported for proxy 192.168.1.0/255.255.255.0/0/0
*Mar 1 00:02:34.567: IPSEC(validate_proposal): proposal doesn't match!
*Mar 1 00:02:34.568: IPSEC(create_sa): SA created with (0x1234, 0x5678) but no inbound or outbound SPI
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The transform set on the remote peer does not match the local transform set, causing IPsec SA negotiation to fail.
The debug output shows that the IPsec SA request is generated, but the transform proposal (esp-3des esp-sha-hmac) is rejected with 'not supported for proxy'. This indicates that the remote peer does not support or is configured with a different transform set, causing the proposal to fail validation. The SA is created with placeholder SPIs (0x1234, 0x5678) but no actual inbound/outbound SPI, confirming negotiation failure due to transform set mismatch.
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 IPsec SA is established successfully with ESP 3DES and SHA.
Why it's wrong here
The output says 'proposal doesn't match' and 'no inbound or outbound SPI', indicating failure.
✓
The transform set on the remote peer does not match the local transform set, causing IPsec SA negotiation to fail.
Why this is correct
The debug explicitly states the proposal is not supported, meaning a mismatch in transform set.
Related concept
Read the scenario before looking for a memorised answer.
✗
The ACL for the crypto map is misconfigured, causing the traffic to not match.
Why it's wrong here
The debug shows src_proxy and dst_proxy, indicating the ACL matched, but the transform set is the issue.
✗
The IKE phase 1 SA is not established before IPsec negotiation.
Why it's wrong here
The debug does not mention IKE phase 1; it focuses on IPsec transform validation.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between IKE phase 1 and phase 2 failures; the trap here is that candidates might assume the SA creation message with SPIs indicates success, but the 'no inbound or outbound SPI' line reveals the SA is incomplete, pointing to a transform set mismatch rather than ACL or IKE phase 1 issues.
Trap categories for this question
Command / output trap
The output says 'proposal doesn't match' and 'no inbound or outbound SPI', indicating failure.
Detailed technical explanation
How to think about this question
IPsec transform sets define the encryption and authentication algorithms (e.g., esp-3des, esp-sha-hmac) and must match exactly on both peers for IPsec SA negotiation to succeed. The 'validate_proposal' step occurs during Quick Mode (IKE phase 2), where the initiator sends a transform set; if the responder does not have a matching transform set, it sends a 'not supported' notification, and the SA creation fails. In real-world scenarios, this often happens when one peer is configured with AES and the other with 3DES, or when one uses SHA-1 and the other SHA-256.
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.
IPsec Site-to-Site VPN — This question tests IPsec Site-to-Site VPN — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The transform set on the remote peer does not match the local transform set, causing IPsec SA negotiation to fail. — The debug output shows that the IPsec SA request is generated, but the transform proposal (esp-3des esp-sha-hmac) is rejected with 'not supported for proxy'. This indicates that the remote peer does not support or is configured with a different transform set, causing the proposal to fail validation. The SA is created with placeholder SPIs (0x1234, 0x5678) but no actual inbound/outbound SPI, confirming negotiation failure due to transform set mismatch.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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