Question 799 of 2,152

BGP Troubleshooting →hardMultiple ChoiceObjective-mapped

IPsec Transform Set Mismatch — Order of Protocols | Cisco CCNP ENARSI 300-410 Explained

This 300-410 practice question tests your understanding of bgp troubleshooting. 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 with a transform set that includes ESP-SHA-HMAC and ESP-AES-256. The VPN tunnel fails to establish, and debug shows 'transform set mismatch'. What 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 peer has the transform set with the same protocols but in a different order (ESP-AES-256 then ESP-SHA-HMAC), which is considered a mismatch.
IPsec transform sets are matched exactly, including the order of protocols.
B
The peer uses ESP-AES-256 with a different key length, such as 128-bit.
Why wrong: AES-256 implies 256-bit key; AES-128 would be a different transform.
C
The peer has 'crypto ipsec transform-set' with 'esp-sha-hmac' and 'esp-aes 256' but also includes 'comp-lzs'.
Why wrong: Extra protocols would also cause a mismatch, but the order issue is more subtle and common.
D
The peer uses 'esp-sha-hmac' as authentication and 'esp-aes 256' as encryption, but the mode is set to transport instead of tunnel.
Why wrong: Mode mismatch is a separate issue and would show a different debug message.

Full breakdown with real-world context →

Quick Answer

The answer is that the IPsec transform set mismatch occurs because the peer lists the same protocols—ESP-AES-256 and ESP-SHA-HMAC—but in a different order, which the Cisco IOS treats as a completely different transform set. During IPsec phase 2 negotiation, the transform set must match exactly on both peers, including the sequence of encryption and authentication protocols; swapping ESP-SHA-HMAC (authentication) before ESP-AES-256 (encryption) breaks the match, even though the individual algorithms are identical. On the Cisco CCNP ENARSI 300-410 exam, this is a classic trap designed to test your understanding that transform sets are order-sensitive, not just content-sensitive—a common misstep when troubleshooting VPN tunnels. Remember the memory tip: "Order is authority" for transform sets; if the peer lists encryption first, you must mirror that sequence exactly, or debug will always show a mismatch.

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 peer has the transform set with the same protocols but in a different order (ESP-AES-256 then ESP-SHA-HMAC), which is considered a mismatch.

The transform set must match exactly on both peers, including the order of protocols. ESP-SHA-HMAC is an authentication protocol, and ESP-AES-256 is encryption. If the peer has them in a different order or uses a different combination, the negotiation fails.

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 peer has the transform set with the same protocols but in a different order (ESP-AES-256 then ESP-SHA-HMAC), which is considered a mismatch.
Why this is correct
IPsec transform sets are matched exactly, including the order of protocols.
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 peer uses ESP-AES-256 with a different key length, such as 128-bit.
Why it's wrong here
AES-256 implies 256-bit key; AES-128 would be a different transform.
✗
The peer has 'crypto ipsec transform-set' with 'esp-sha-hmac' and 'esp-aes 256' but also includes 'comp-lzs'.
Why it's wrong here
Extra protocols would also cause a mismatch, but the order issue is more subtle and common.
✗
The peer uses 'esp-sha-hmac' as authentication and 'esp-aes 256' as encryption, but the mode is set to transport instead of tunnel.
Why it's wrong here
Mode mismatch is a separate issue and would show a different debug message.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.

Trap categories for this question

Command / output trap
Mode mismatch is a separate issue and would show a different debug message.

Detailed technical explanation

How to think about this question

This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.

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.
Use explanations to understand the rule behind the answer.

TExam Day Tips

Underline the problem statement mentally.
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 300-410 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.

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FAQ

Questions learners often ask

What does this 300-410 question test?

BGP Troubleshooting — This question tests BGP Troubleshooting — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The peer has the transform set with the same protocols but in a different order (ESP-AES-256 then ESP-SHA-HMAC), which is considered a mismatch. — The transform set must match exactly on both peers, including the order of protocols. ESP-SHA-HMAC is an authentication protocol, and ESP-AES-256 is encryption. If the peer has them in a different order or uses a different combination, the negotiation fails.

What should I do if I get this 300-410 question wrong?

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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