Question 748 of 1,958
easyMultiple ChoiceObjective-mapped
350-401 Practice Question: An engineer is troubleshooting a site-to-site VPN…
An engineer is troubleshooting a site-to-site VPN between a Cisco ASA and a Cisco IOS router. The VPN is configured using IKEv1 with pre-shared keys. The tunnel establishes and traffic flows, but after a few hours, the tunnel drops and re-establishes. The engineer checks the logs and sees that the Phase 1 SA is being rekeyed. What is the most likely reason for the tunnel dropping?
⚠ Common exam trap
Cisco often tests the misconception that a short DPD interval causes rekeys, but the key clue is the log entry specifically mentioning 'Phase 1 SA being rekeyed,' which directly points to the IKE lifetime, not DPD.
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 IKE Phase 1 lifetime is set too low.
The most likely reason for the tunnel dropping and re-establishing after a few hours is that the IKE Phase 1 lifetime is set too low. When the Phase 1 SA expires, it must be rekeyed, which can cause a brief interruption in the VPN tunnel. The logs confirm a Phase 1 SA rekey, directly pointing to a short lifetime as the root cause.
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 Dead Peer Detection (DPD) interval is too short.
Why it's wrong here
DPD detects failures, but does not cause drops by itself.
- ✓
The IKE Phase 1 lifetime is set too low.
Why this is correct
Correct because a short lifetime causes frequent rekeys, which can lead to drops if not synchronized.
- ✗
The IPsec transform set is misconfigured.
Why it's wrong here
Transform set issues would prevent tunnel establishment.
- ✗
The Phase 2 lifetime is longer than Phase 1.
Why it's wrong here
Phase 2 lifetime being longer does not cause drops.
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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Last reviewed: Jul 4, 2026
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