hardMultiple ChoiceObjective-mapped
350-701 Practice Question: An engineer is troubleshooting an IPsec VPN…
An engineer is troubleshooting an IPsec VPN between two Cisco routers. The tunnel is up, but traffic is not passing. The encryption domain on both sides is correctly configured. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the distinction between tunnel establishment (IKE phase 1 and 2) and data-plane forwarding, tricking candidates into thinking a tunnel being 'up' guarantees traffic flow, when in fact ACLs or firewall rules can block the encrypted payload.
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
✓
ACL on the WAN interface blocking ESP traffic
When the IPsec tunnel is up but no traffic passes, the most common cause is that the WAN interface ACL is blocking ESP (protocol 50) or UDP/4500 (NAT-T) traffic. Even though IKE and IPsec SAs are established, if the ACL drops the encrypted packets, the tunnel appears operational but cannot forward data. This is distinct from transform set or IKE mismatches, which would prevent the tunnel from coming up at all.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Mismatched IPSec transform sets
Why it's wrong here
If transform sets mismatched, the IPsec SA would not be established.
- ✗
Routing loop
Why it's wrong here
A routing loop is unlikely if the tunnel is up; routing should work normally.
- ✓
ACL on the WAN interface blocking ESP traffic
Why this is correct
ESP traffic (IP protocol 50) may be dropped by an inbound or outbound ACL.
- ✗
Mismatched IKE phase 1 parameters
Why it's wrong here
If IKE parameters mismatched, the tunnel would not come up.
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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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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