The answer is incorrect routing entries in the VPN routing table. When an IPsec VPN tunnel shows established but no traffic passes, it means IKE Phase 1 and Phase 2 completed successfully, so authentication and encryption settings are correct; the issue lies in the control plane, not the data plane. Specifically, the branch office lacks a route directing the destination subnet to the tunnel interface, so packets are sent unencrypted via the physical interface or dropped entirely. On the CISSP exam, this tests your understanding that tunnel establishment and traffic flow are separate—a common trap is assuming a green light means data is moving. Remember the memory tip: “Tunnel up, route down” to distinguish between a functional security association and a missing path.
CISSP Communication and Network Security Practice Question
This CISSP practice question tests your understanding of communication and network security. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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.
An organization is implementing IPsec VPN tunnels between multiple branch offices and the main office. The security team notices that the VPN tunnels are established successfully but no traffic passes through. Which of the following is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Incorrect routing entries in the VPN routing table
When IPsecVPN tunnels are established (IKE Phase 1 and Phase 2 complete) but no traffic passes, the most common cause is incorrect routing entries in the VPN routing table. The tunnel may be up, but if the branch office does not have a route pointing the destination subnet to the tunnel interface (e.g., a static route or policy-based route), packets will be sent out the physical interface unencrypted or dropped. This is distinct from authentication or encryption mismatches, which would prevent tunnel establishment entirely.
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.
✗
Certificates expired
Why it's wrong here
Expired certificates would cause authentication failure during Phase 1, preventing tunnel establishment.
✗
Mismatched encryption algorithm
Why it's wrong here
Encryption algorithm mismatch would also prevent tunnel establishment, typically causing Phase 2 failure.
✓
Incorrect routing entries in the VPN routing table
Why this is correct
If routes are missing or misconfigured, traffic will not be forwarded into the tunnel even though the tunnel is operational.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Mismatched IKE version
Why it's wrong here
IKE version mismatch would prevent Phase 1 from completing; the tunnel would not be established.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume a successful tunnel establishment (Phase 1 and Phase 2 UP) guarantees traffic flow, but ISC2 often tests the subtle distinction between control plane success and data plane forwarding, where routing misconfigurations silently drop traffic.
Detailed technical explanation
How to think about this question
In IPsec VPNs, the routing table must contain a route for the remote subnet that points to the virtual tunnel interface (e.g., 'ip route 192.168.2.0 255.255.255.0 Tunnel0'). Without this, the router performs a standard FIB lookup and forwards traffic out the physical interface, bypassing the IPsec crypto map. This is a common misconfiguration in hub-and-spoke topologies where the spoke has a default route to the internet but no specific route for the hub's internal networks.
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 developer is choosing between AES-256 (symmetric) and RSA-2048 (asymmetric) for encrypting a large file that will be sent to a partner. Symmetric encryption is fast but requires key exchange; asymmetric is slower but solves the key distribution problem. A hybrid approach — encrypt the file with AES, encrypt the AES key with RSA — is standard. Questions like this test whether you understand when each approach applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CISSP question in full detail.
Communication and Network Security — This question tests Communication and Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Incorrect routing entries in the VPN routing table — When IPsec VPN tunnels are established (IKE Phase 1 and Phase 2 complete) but no traffic passes, the most common cause is incorrect routing entries in the VPN routing table. The tunnel may be up, but if the branch office does not have a route pointing the destination subnet to the tunnel interface (e.g., a static route or policy-based route), packets will be sent out the physical interface unencrypted or dropped. This is distinct from authentication or encryption mismatches, which would prevent tunnel establishment entirely.
What should I do if I get this CISSP question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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