easyMultiple Choice
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
Dead Peer Detection (DPD) is a keepalive mechanism that sends R-U-THERE probes to confirm that the remote peer is still alive; it does not initiate rekeying or alter SA lifetimes. A short DPD interval simply causes more frequent probes, and an SA is only marked dead after several unanswered retries. For a live peer with normal latency, shorter DPD timing will not drop traffic; if false positives occur, they are due to aggressive retry handling, not the DPD interval itself. Therefore, this option cannot explain periodic tunnel drops.
- ✓
The IKE Phase 1 lifetime is set too low.
Why this is correct
An IKE Phase 1 lifetime that is set too low forces frequent ISAKMP SA renegotiations. If the new Phase 1 exchange cannot complete before the current Phase 1 SA expires, the existing IPsec SAs are invalidated and traffic is dropped while new tunnels are built. Even when rekeys do complete, the short lifetime increases the overlap and timing mismatch with the Phase 2 IPsec lifetime, making drops more likely during transitions. This is the correct cause because it directly matches the symptom of recurring connectivity interruptions.
- ✗
The IPsec transform set is misconfigured.
Why it's wrong here
A misconfigured IPsec transform set results in a failure to find a matching security proposal during negotiation, which prevents the IKEv2/IPsec SA from being established in the first place. For example, if the two peers select different ESP encryption algorithms or hash functions, the tunnel either stays down or appears only after manual reconfiguration. Intermittent drops that recover on their own are not the signature of a transform mismatch; configuration errors are constant until fixed. Thus, this wrong answer describes an establishment problem, not a lifetime or rekey issue.
- ✗
The Phase 2 lifetime is longer than Phase 1.
Why it's wrong here
Setting the Phase 2 (IPsec) lifetime longer than the Phase 1 (ISAKMP) lifetime is not inherently disruptive; both SAs remain valid until their individually configured expiration times. When the Phase 1 SA rekeys earlier, the Phase 2 SA continues to protect traffic independently, and when the Phase 2 SA eventually expires, a fresh Phase 1 SA is already available if the rekey was successful. Even in the worst case, the router would first rebuild Phase 1 and then renegotiate Phase 2, which is normal behavior and does not cause catastrophic drops. The relative ordering of lifetimes alone does not force the tunnel down.
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 by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.