Question 66 of 2,015

WAN Technologies →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is a mismatch in the IPsec transform set, specifically that the local router uses ESP with SHA-1 while the remote router expects AES-GCM. This is correct because even when IKEv2 Phase 1 is up, Phase 2 negotiations will fail if the transform set parameters—such as encryption algorithm, integrity algorithm, or encapsulation mode—do not match between peers, preventing the IPsec SA from being established and causing zero encapsulated packets. On the ENCOR 350-401 exam, this scenario tests your ability to distinguish between Phase 1 and Phase 2 failures; a common trap is assuming the ACL is the problem when it is correctly configured, but the real issue lies in mismatched transform sets or IKEv2 proposals. Remember: Phase 1 is about authentication and key exchange, Phase 2 is about protecting traffic—if the algorithms don’t align, the tunnel stays dark. Memory tip: “Phase 2 is picky—match your crypto, or your packets stay sticky.”

CCNP WAN Technologies Practice Question

This 350-401 practice question tests your understanding of wan technologies. 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 a site-to-site IPsec VPN tunnel between two Cisco routers. The tunnel is established and IKEv2 Phase 1 is up, but no traffic passes. The engineer checks the crypto map and sees that the ACL is configured to permit traffic between the two LAN subnets. However, 'show crypto ipsec sa' shows that the number of packets encapsulated and decapsulated is zero. 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.

Question 1mediummultiple choice

Study the full ACL explanation →

A
The crypto map is not applied to the correct interface.
Why wrong: Incorrect. If the crypto map were not applied, Phase 1 would not come up. Phase 1 is up, so the crypto map is applied correctly.
B
The IPsec transform set uses ESP with SHA-1, but the remote router expects AES-GCM.
Correct. A mismatch in the transform set (e.g., encryption or authentication algorithms) will prevent Phase 2 from establishing, even though Phase 1 (which uses a different proposal) may succeed.
C
The ACL on the crypto map is missing the 'permit ip' statement for the return traffic.
Why wrong: Incorrect. The ACL only needs to match the traffic that will be encrypted; it is not required to permit return traffic explicitly because IPsec uses the same SA for both directions if configured correctly.
D
The tunnel interface is down due to a routing issue.
Why wrong: Incorrect. The tunnel interface is not used in a crypto map-based VPN; it is a site-to-site VPN using physical interfaces. The tunnel interface is not relevant here.

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 IPsec transform set uses ESP with SHA-1, but the remote router expects AES-GCM.

When IKEv2 Phase 1 is up but Phase 2 (IPsec SA) is not established, the most common cause is a mismatch in the proxy identities (the interesting traffic ACL) or a mismatch in the IPsec transform set parameters. Since the ACL is configured correctly, the issue is likely a mismatch in the transform set or the IKEv2 proposal.

Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.

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 crypto map is not applied to the correct interface.
Why it's wrong here
Incorrect. If the crypto map were not applied, Phase 1 would not come up. Phase 1 is up, so the crypto map is applied correctly.
✓
The IPsec transform set uses ESP with SHA-1, but the remote router expects AES-GCM.
Why this is correct
Correct. A mismatch in the transform set (e.g., encryption or authentication algorithms) will prevent Phase 2 from establishing, even though Phase 1 (which uses a different proposal) may succeed.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The ACL on the crypto map is missing the 'permit ip' statement for the return traffic.
Why it's wrong here
Incorrect. The ACL only needs to match the traffic that will be encrypted; it is not required to permit return traffic explicitly because IPsec uses the same SA for both directions if configured correctly.
✗
The tunnel interface is down due to a routing issue.
Why it's wrong here
Incorrect. The tunnel interface is not used in a crypto map-based VPN; it is a site-to-site VPN using physical interfaces. The tunnel interface is not relevant here.

Common exam traps

Common exam trap: usable hosts are not the same as total addresses

Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.

Detailed technical explanation

How to think about this question

Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.

KKey Concepts to Remember

CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.

TExam Day Tips

Write the block size before choosing the subnet.
Check whether the question asks for hosts, subnets or a specific address range.
Do not confuse /24, /25, /26 and /27 host counts.

Key takeaway

Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.

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.

What to study next

Got this wrong? Here's your next step.

Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 350-401 subnetting questions on CIDR, address ranges, and subnet selection.

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FAQ

Questions learners often ask

What does this 350-401 question test?

WAN Technologies — This question tests WAN Technologies — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: The IPsec transform set uses ESP with SHA-1, but the remote router expects AES-GCM. — When IKEv2 Phase 1 is up but Phase 2 (IPsec SA) is not established, the most common cause is a mismatch in the proxy identities (the interesting traffic ACL) or a mismatch in the IPsec transform set parameters. Since the ACL is configured correctly, the issue is likely a mismatch in the transform set or the IKEv2 proposal.

What should I do if I get this 350-401 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?

CIDR notation defines the prefix length.

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