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350-701 Practice Question: A company has a site-to-site VPN between two ASA…
A company has a site-to-site VPN between two ASA firewalls using IKEv2. The tunnel was working but after an upgrade, it fails. The engineer verifies that the pre-shared keys match, IKE proposals are compatible, and the crypto ACL is correctly defined. What is the next likely cause to investigate?
⚠ Common exam trap
Cisco often tests the misconception that post-upgrade failures are always due to configuration mismatches, but the trap here is that intermediate firewall rules or ACL changes are frequently overlooked after an upgrade, even when all other parameters are verified as correct.
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 firewall rules on the intermediate devices are blocking ISAKMP traffic.
The tunnel was working before the upgrade and the engineer has already verified that the pre-shared keys, IKE proposals, and crypto ACL are correct. After an ASA upgrade, intermediate firewall rules or ACLs may be reset or changed, potentially blocking ISAKMP (UDP 500/4500) traffic. Since the tunnel fails to establish, the next logical step is to check if ISAKMP traffic is being permitted through all intermediate devices, as this is a common post-upgrade issue.
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 firewall rules on the intermediate devices are blocking ISAKMP traffic.
Why this is correct
Intermediate firewall changes during upgrade can block UDP ports 500 and 4500, preventing IKE negotiation. This is a common cause.
- ✗
The ACL for interesting traffic is missing.
Why it's wrong here
If the crypto ACL were missing, the tunnel would not have been triggered. But since it was working, it's likely still present.
- ✗
The crypto map is not applied to the correct interface.
Why it's wrong here
If the crypto map were not applied, the tunnel would not have been working before the upgrade. Since it was working, the map application is likely correct.
- ✗
The MTU is too high.
Why it's wrong here
MTU issues can cause problems but typically affect traffic after tunnel is up, not tunnel establishment.
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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