350-401 Security Practice Question
An engineer is configuring an IPsec site-to-site VPN between two Cisco IOS XE routers. Phase 1 completes successfully, but Phase 2 fails and no interesting traffic is encrypted. The engineer confirms that the ACLs on both peers mirror each other correctly. Which configuration element should be verified next to resolve the Phase 2 failure?
⚠ Common exam trap
The trap here is revisiting Phase 1 parameters such as the pre-shared key after Phase 1 has already succeeded, instead of focusing on Phase 2 elements like the transform set.
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
✓
Verify that the transform set on both peers specifies matching encryption and integrity algorithms for Phase 2.
IPsec Phase 2 negotiation, also called quick mode, relies on matching transform sets that define the encryption and integrity algorithms for the data SA. When Phase 1 succeeds but Phase 2 fails, mismatched transform sets are a common cause. Verifying that both peers use identical transform set definitions resolves the negotiation failure.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Verify that the Diffie-Hellman group in the ISAKMP policy matches the group used in the transform set.
Why it's wrong here
Diffie-Hellman groups are specified in the ISAKMP policy for Phase 1 and optionally in the transform set for perfect forward secrecy in Phase 2. The transform set itself does not require a Diffie-Hellman group unless PFS is configured. The described mismatch is a misunderstanding of where DH groups are defined.
- ✗
Verify that the crypto map sequence number is higher on the responding peer than on the initiating peer.
Why it's wrong here
Crypto map sequence numbers determine the order in which map entries are evaluated locally and do not need to match or differ in a specific way between peers. They are not negotiated during Phase 2. This check is irrelevant to the quick mode failure described in the scenario.
- ✗
Verify that the ISAKMP policy on both peers uses the same pre-shared key for Phase 1 authentication.
Why it's wrong here
The pre-shared key is used during Phase 1 authentication. Because the scenario states Phase 1 completes successfully, the ISAKMP policy and pre-shared key are already working. Changing or verifying them further will not resolve a Phase 2 failure, which depends on the transform set and proxy identities.
- ✓
Verify that the transform set on both peers specifies matching encryption and integrity algorithms for Phase 2.
Why this is correct
Phase 2 negotiation uses the transform set to define encryption and integrity algorithms for the IPsec SA. If the transform sets differ between peers, the quick mode negotiation fails even though Phase 1 succeeded. Matching transform sets on both routers is essential to complete Phase 2 and establish the data-protection tunnel.
Visual reference
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.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
This 350-401 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-401 exam.