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CISSP Practice Question: Is implementing IPsec VPN tunnels between…
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?
⚠ Common exam trap
It's easy for candidates to 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.
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
✓
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.
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
When certificates are employed for authentication during IKE Phase 1, an expired certificate is immediately deemed invalid by the peer device. This invalidation prevents successful cryptographic authentication, leading to the termination of the Phase 1 negotiation. Consequently, the ISAKMP Security Association (SA) cannot be established, meaning the VPN tunnel will not come up at all, and no traffic can pass.
- ✗
Mismatched encryption algorithm
Why it's wrong here
During IKE Phase 2, the VPN peers negotiate the specific Security Association (SA) for IPSec itself, which includes the encryption algorithm (e.g., AES-256, 3DES) to protect the actual data traffic. If the proposed encryption algorithms do not match or are not mutually acceptable, the Phase 2 negotiation will fail. This prevents the IPSec SA from being established, meaning the tunnel cannot be fully brought up to encapsulate and encrypt user data.
- ✓
Incorrect routing entries in the VPN routing table
Why this is correct
A VPN tunnel can be successfully established, meaning IKE Phase 1 and Phase 2 complete, and the Security Associations are active and operational. However, if the local routing table on the VPN gateway lacks specific routes directing traffic destined for the remote network *into* the VPN tunnel interface or crypto map, the traffic will simply bypass the tunnel or be dropped. The tunnel itself is functional, but the network device doesn't know to use it for the intended traffic.
- ✗
Mismatched IKE version
Why it's wrong here
IKE (Internet Key Exchange) has distinct versions, primarily IKEv1 and IKEv2, which are fundamentally incompatible protocols for establishing the initial secure channel. If one VPN peer is configured to use IKEv1 and the other is configured for IKEv2, they cannot understand each other's negotiation messages or security parameters. This fundamental protocol mismatch will cause Phase 1 negotiation to fail immediately, preventing the establishment of the ISAKMP SA and thus the entire VPN tunnel.
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.
Go deeper
Related to this question
Learn chapter
Security Governance and Principles
Key term
Authentication
Authentication is the process of verifying that someone or something is who or what it claims to be before granting access to a system or resource.
Key term
Policy
A policy is a set of rules or guidelines that defines how an organization manages, secures, and operates its IT systems and services.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This CISSP practice question is part of Courseiva's free ISC2 certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CISSP exam.