mediumMultiple ChoiceObjective-mapped
300-410 Practice Question: Runs the following command on Router R9: R9# show…
A network engineer runs the following command on Router R9:
R9# show logging | include %DMVPN-5-ADJCHG *Mar 1 00:01:05.123: %DMVPN-5-ADJCHG: NHRP: Peer 10.0.0.1 on Tunnel0 is UP *Mar 1 00:02:10.456: %DMVPN-5-ADJCHG: NHRP: Peer 10.0.0.1 on Tunnel0 is DOWN *Mar 1 00:03:15.789: %DMVPN-5-ADJCHG: NHRP: Peer 10.0.0.1 on Tunnel0 is UP *Mar 1 00:04:20.012: %DMVPN-5-ADJCHG: NHRP: Peer 10.0.0.1 on Tunnel0 is DOWN
Based on this output, what is the most likely problem?
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 DMVPN tunnel is flapping due to an unstable IPsec SA.
The output shows NHRP peer flapping on Tunnel0, indicating that the DMVPN tunnel is unstable. This is often due to a flaking physical interface, IPsec SA expiration, or routing issues. The peer 10.0.0.1 is going up and down repeatedly.
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 DMVPN tunnel is flapping due to an unstable IPsec SA.
Why this is correct
DMVPN uses IPsec for encryption; if the IPsec SA is not established or is flapping, the NHRP peer will go up and down.
- ✗
The NHRP hold time is set too high.
Why it's wrong here
A high hold time would make the peer take longer to go down, but the pattern shows quick flapping.
- ✗
The router has a mismatched DMVPN phase configuration.
Why it's wrong here
Mismatched phases would prevent the peer from coming up at all.
- ✗
The tunnel interface is administratively down.
Why it's wrong here
If the tunnel were administratively down, no peers would come up.
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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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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