300-410 VPN Technologies Practice Question
A network administrator is troubleshooting an IPsec VPN tunnel between two Cisco IOS routers that uses IKEv2. Phase 1 is up, but Phase 2 fails. The administrator reviews the configuration and notices that the transform set on one router includes esp-aes 256 esp-sha256-hmac, while the other router has esp-aes 256 esp-sha512-hmac. The administrator wants to ensure the Phase 2 SA is established. Which action should the administrator take?
⚠ Common exam trap
The trap here is assuming that Phase 2 failures are caused by Phase 1 parameters like DH group or IKEv2 proposal settings, rather than the transform set 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
✓
Configure a matching transform set on both routers with the same encryption and integrity algorithms.
IPsec Phase 2 requires both peers to agree on a transform set that defines the encryption and integrity algorithms. The mismatch between esp-sha256-hmac and esp-sha512-hmac means the peers cannot agree on the integrity algorithm, so the IPsec SA fails to establish. The administrator must configure a matching transform set on both routers, ensuring the encryption and integrity algorithms are identical, to allow Phase 2 to complete.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Configure a matching transform set on both routers with the same encryption and integrity algorithms.
Why this is correct
Phase 2 (IPsec SA) requires both peers to agree on the transform set, which specifies the encryption and integrity algorithms. The mismatch between esp-sha256-hmac and esp-sha512-hmac prevents the IPsec SA from being established. Configuring identical transform sets on both routers, such as esp-aes 256 esp-sha256-hmac, ensures both peers propose and accept the same algorithms, allowing Phase 2 to complete successfully.
- ✗
Change the IKEv2 proposal to use a different DH group.
Why it's wrong here
The IKEv2 proposal is used during Phase 1 to establish the IKE SA. Since Phase 1 is already up, the DH group in the proposal is not the cause of the Phase 2 failure. Modifying the IKEv2 proposal would affect Phase 1 and could disrupt the existing IKE SA, but it would not resolve the transform set mismatch that is blocking the IPsec SA. Therefore, this action is not appropriate.
- ✗
Restart the IKEv2 process on both routers to clear the Phase 1 SA and force renegotiation.
Why it's wrong here
Restarting the IKEv2 process would clear the IKE SA and force Phase 1 to renegotiate, but it would not change the transform set mismatch. The same Phase 2 failure would occur because the peers would still propose different integrity algorithms. Clearing SAs is a troubleshooting step, but it does not resolve the underlying configuration mismatch, so it is not a valid solution.
- ✗
Enable perfect forward secrecy on both routers by configuring the same DH group in the IPsec profile.
Why it's wrong here
Perfect forward secrecy (PFS) adds a DH exchange to Phase 2, but it requires the DH group to match in the IPsec profile. While PFS is a Phase 2 parameter, the failure here is due to mismatched integrity algorithms in the transform set, not PFS. Enabling PFS with the same DH group would not fix the transform set mismatch. The administrator must first align the transform sets; PFS is a separate consideration.
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
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