hardMultiple ChoiceObjective-mapped
300-410 Practice Question: Is troubleshooting an IPsec site-to-site VPN…
A network engineer is troubleshooting an IPsec site-to-site VPN between two Cisco routers. The tunnel is up, but traffic intermittently drops. The engineer notices that the 'show crypto ipsec sa' output shows the packet counters incrementing for both encrypt and decrypt, but the 'pkts encaps failed' counter is also increasing. What is the most likely cause?
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 crypto map is not applied to the correct interface.
The 'pkts encaps failed' counter indicates that the router is unable to encrypt packets that should be encrypted. This typically happens when the crypto map's access list matches traffic, but the route to the remote LAN points out an interface that does not have the crypto map applied, causing the router to try to send the packet without encryption.
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 crypto map is not applied to the correct interface.
Why this is correct
Correct because if the route to the remote LAN points to an interface without the crypto map, the router will attempt to send the packet unencrypted, resulting in encapsulation failure.
- ✗
The IPsec transform set is misconfigured with incompatible algorithms.
Why it's wrong here
Incorrect because if the transform set were incompatible, the tunnel would not be up and SAs would not be established.
- ✗
The IKE keepalive timer is too short, causing frequent rekeying.
Why it's wrong here
Incorrect because keepalive timers affect session stability but do not cause encapsulation failures.
- ✗
The MTU on the outside interface is too small, causing fragmentation.
Why it's wrong here
Incorrect because fragmentation issues would typically show as 'pkts fragmented' or 'pkts not fragmented' counters, not encapsulation failures.
Visual reference
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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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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