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 where the tunnel is up and traffic is flowing, but the engineer notices that the 'show crypto ipsec sa' output shows the 'pkts encaps failed' counter incrementing slowly over time. The tunnel remains up. 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
✓
There is a floating static route that points to the physical interface instead of the tunnel for some subnets.
A slow but steady increase in 'pkts encaps failed' indicates that some packets that should be encrypted are not being encrypted. This is often caused by a routing issue where some traffic to the remote LAN is being routed out an interface that does not have the crypto map, bypassing 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.
- ✓
There is a floating static route that points to the physical interface instead of the tunnel for some subnets.
Why this is correct
Correct because if a floating static route (with a higher administrative distance) becomes active for some traffic, those packets will be sent out the physical interface without encryption, causing encapsulation failures.
- ✗
The crypto map is applied to the tunnel interface instead of the physical interface.
Why it's wrong here
Incorrect because applying the crypto map to the tunnel interface would cause all tunnel traffic to fail, not just some packets.
- ✗
The IPsec SA lifetime is set too high.
Why it's wrong here
Incorrect because SA lifetime does not cause encapsulation failures.
- ✗
The access list in the crypto map is too restrictive and missing some subnets.
Why it's wrong here
Incorrect because if the access list were missing subnets, those packets would not be matched and would be sent unencrypted, but the 'pkts encaps failed' counter specifically counts packets that are matched but cannot be encrypted due to routing issues.
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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