The answer is to investigate whether intermediate firewall rules are blocking ISAKMP traffic. This is correct because, after an ASA upgrade, default ACLs or firewall policies on intermediate devices can be reset or overwritten, silently dropping the UDP 500 and 4500 packets required for IKEv2 tunnel establishment—even when peer configurations like pre-shared keys, proposals, and crypto ACLs remain valid. On the Cisco SCOR 350-701 exam, this scenario tests your understanding that VPN failures are not always a misconfiguration of the tunnel endpoints; a common trap is to recheck the same settings repeatedly while overlooking the data path. The exam often presents a working-then-broken upgrade scenario to see if you remember that intermediate stateful firewalls or routers with inspect policies may block ISAKMP after a reboot or firmware change. Memory tip: “After an upgrade, check the path—UDP 500 and 4500 must survive the firewall gauntlet.”
350-701 Network Security Practice Question
This 350-701 practice question tests your understanding of network security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
The firewall rules on the intermediate devices are blocking ISAKMP traffic.
Intermediate firewall changes during upgrade can block UDP ports 500 and 4500, preventing IKE negotiation. This is a common cause.
B
The ACL for interesting traffic is missing.
Why wrong: If the crypto ACL were missing, the tunnel would not have been triggered. But since it was working, it's likely still present.
C
The crypto map is not applied to the correct interface.
Why wrong: 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.
D
The MTU is too high.
Why wrong: MTU issues can cause problems but typically affect traffic after tunnel is up, not tunnel establishment.
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 correct answer is A because 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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Related concept
Read the scenario before looking for a memorised answer.
✗
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Detailed technical explanation
How to think about this question
IKEv2 uses UDP port 500 for initial negotiation and UDP port 4500 for NAT traversal. After an ASA upgrade, default ACLs or interface access rules may be overwritten or reset, especially if the upgrade process modifies the running configuration. A common real-world scenario is that an upgrade resets the 'access-group' applied to the outside interface, causing ISAKMP packets to be dropped silently, which prevents Phase 1 (IKE_SA_INIT) from completing.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network Security — This question tests Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The firewall rules on the intermediate devices are blocking ISAKMP traffic. — The correct answer is A because 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.
What should I do if I get this 350-701 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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