hardMultiple ChoiceObjective-mapped
300-410 Practice Question: Is troubleshooting a DMVPN phase 2 network where…
A network engineer is troubleshooting a DMVPN phase 2 network where spoke-to-spoke tunnels are established, but traffic between spokes is intermittently dropped. The engineer captures packets and sees that IPsec packets are being fragmented. The tunnel interface MTU is set to 1400 bytes, and the physical interface MTU is 1500 bytes. The engineer also notices that the IPsec transform set uses ESP with AES-256 and SHA-256. What is the most likely cause of the intermittent drops?
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 tunnel MTU is set too high for the IPsec overhead, causing fragmentation and potential drops.
IPsec adds overhead (ESP header, trailer, and authentication data). With AES-256 and SHA-256, the overhead can be around 50-60 bytes. If the tunnel MTU is set to 1400, the effective payload MTU is lower. Fragmentation can cause drops if the DF bit is set or if intermediate routers drop fragments. The issue is that the tunnel MTU is too high for the IPsec overhead, causing fragmentation.
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 IPsec transform set uses AES-256, which requires more CPU and causes performance drops.
Why it's wrong here
Incorrect because CPU load does not cause intermittent drops; fragmentation is the issue.
- ✓
The tunnel MTU is set too high for the IPsec overhead, causing fragmentation and potential drops.
Why this is correct
Correct because the tunnel MTU of 1400 bytes does not account for IPsec overhead, leading to fragmentation and drops.
- ✗
The physical interface MTU is set to 1500, which is too high for DMVPN.
Why it's wrong here
Incorrect because the physical interface MTU is standard; the tunnel MTU should be lower to accommodate IPsec overhead.
- ✗
The spokes have different IPsec transform sets configured.
Why it's wrong here
Incorrect because mismatched transform sets would prevent IPsec negotiation, not cause intermittent 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.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
DMVPN Phase 2
DMVPN Phase 2 is an advanced Cisco routing technology that allows spoke routers to communicate directly with one another without sending traffic through a central hub, using dynamic routing protocols and multipoint GRE tunnels.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 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 300-410 exam.