Question 1,917 of 2,152

Device Management →hardMultiple ChoiceObjective-mapped

300-410 Device Management Practice Question

This 300-410 practice question tests your understanding of device management. 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 configures IPsec between two routers using transform-set esp-aes 256 esp-sha-hmac. The tunnel fails to establish. Debug shows 'transform set proposal mismatch'. Which is the most likely explanation?

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.

A
The other router uses 'esp-aes' without specifying the key length, defaulting to 128-bit, causing a mismatch.
If one side specifies 256-bit and the other defaults to 128-bit, the transform sets do not match.
B
The transform-set uses SHA-1, which is not supported by the other router.
Why wrong: SHA-1 is widely supported; the issue is the AES key length.
C
The IPsec proposal includes both esp-aes and esp-3des, causing confusion.
Why wrong: Only one encryption algorithm is specified in the transform-set.
D
The transform-set is missing the authentication header.
Why wrong: ESP provides both encryption and authentication; AH is not required.

Full breakdown with real-world context →

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

The other router uses 'esp-aes' without specifying the key length, defaulting to 128-bit, causing a mismatch.

The debug output 'transform set proposal mismatch' indicates that the IPsec transform sets on the two peers do not match. When 'esp-aes 256' is configured on one router, the other router must explicitly specify 'esp-aes 256' as well; if it only uses 'esp-aes' without specifying a key length, Cisco IOS defaults to AES-128. This mismatch in encryption algorithm strength (256-bit vs. 128-bit) causes the IKE phase 2 negotiation to fail.

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 other router uses 'esp-aes' without specifying the key length, defaulting to 128-bit, causing a mismatch.
Why this is correct
If one side specifies 256-bit and the other defaults to 128-bit, the transform sets do not match.
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 transform-set uses SHA-1, which is not supported by the other router.
Why it's wrong here
SHA-1 is widely supported; the issue is the AES key length.
✗
The IPsec proposal includes both esp-aes and esp-3des, causing confusion.
Why it's wrong here
Only one encryption algorithm is specified in the transform-set.
✗
The transform-set is missing the authentication header.
Why it's wrong here
ESP provides both encryption and authentication; AH is not required.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the default behavior of 'esp-aes' (which defaults to 128-bit) versus explicit 'esp-aes 256', trapping candidates who assume that 'esp-aes' implies 256-bit or that the key length is negotiated automatically.

Detailed technical explanation

How to think about this question

In Cisco IOS, when configuring 'crypto ipsec transform-set', the encryption algorithm keyword 'esp-aes' without a key length defaults to AES-128, while 'esp-aes 256' explicitly requests 256-bit keys. During IKE phase 2 quick mode, the initiator sends its transform set proposals, and the responder compares them against its own configured transform set; if the encryption algorithm, integrity algorithm, or mode (tunnel/transport) differ, the responder sends a 'transform set proposal mismatch' notification. A real-world scenario where this matters is when templates or automation scripts use 'esp-aes' by default, causing interoperability failures with manually configured peers that specify 'esp-aes 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 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. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.

Quick reference

VPN Protocol Comparison

Protocol	Port	Encryption	Authentication	Use Case
IKEv2 / IPsec	UDP 500 / 4500	AES-256	Certificates / PSK	Site-to-site & remote access
SSL / TLS VPN	TCP 443	TLS 1.3	Certificates / MFA	Clientless remote access
L2TP / IPsec	UDP 1701	AES (IPsec)	PSK / Certificates	Legacy remote access
WireGuard	UDP 51820	ChaCha20	Public keys	Modern high-performance VPN
PPTP	TCP 1723	MPPE (weak)	MS-CHAPv2	Legacy — avoid in production

PPTP is considered insecure. IKEv2/IPsec and SSL VPN are the current recommended options.

What to study next

Got this wrong? Here's your next step.

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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FAQ

Questions learners often ask

What does this 300-410 question test?

Device Management — This question tests Device Management — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The other router uses 'esp-aes' without specifying the key length, defaulting to 128-bit, causing a mismatch. — The debug output 'transform set proposal mismatch' indicates that the IPsec transform sets on the two peers do not match. When 'esp-aes 256' is configured on one router, the other router must explicitly specify 'esp-aes 256' as well; if it only uses 'esp-aes' without specifying a key length, Cisco IOS defaults to AES-128. This mismatch in encryption algorithm strength (256-bit vs. 128-bit) causes the IKE phase 2 negotiation to fail.

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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