Quick Answer
IPsec tunnel mode is the correct choice because it encrypts the entire original IP packet, including its header, and wraps it in a new IP header for secure transport across an untrusted WAN, while also using IKE to mutually authenticate both data center endpoints. This protocol is specifically designed for secure site-to-site VPN connections between data centers, as it provides both confidentiality for all traffic and strong endpoint authentication, meeting the dual requirement of encrypting everything and verifying both sides. On the CISSP exam, this question tests your understanding of network security protocols in the Communication and Network Security domain, where a common trap is confusing IPsec tunnel mode with transport mode—remember that tunnel mode protects the whole packet for gateway-to-gateway links, while transport mode only encrypts the payload for host-to-host. A useful memory tip: think of tunnel mode as a secure armored car that hides the entire package inside a new outer wrapper, whereas transport mode is like a sealed envelope inside a clear bag.
This CISSP practice question tests your understanding of communication and network security. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 network architect is designing a secure connection between two data centers across an untrusted WAN. The requirement is to encrypt all traffic and authenticate both endpoints. Which protocol should be used?
SSH
Why wrong: SSH is for secure terminal access, not site-to-site.
IPsec tunnel mode
IPsec tunnel mode encrypts and authenticates entire packets between gateways.
MPLS
Why wrong: MPLS provides traffic engineering but no encryption.
SSL/TLS
Why wrong: SSL/TLS is not designed for site-to-site VPN; it is for client-server.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
IPsec tunnel mode
IPsec tunnel mode is the correct choice because it encrypts the entire IP packet, including the original IP header, and encapsulates it within a new IP header for secure transport across an untrusted WAN. It also provides mutual authentication of both endpoints using IKE (Internet Key Exchange) with pre-shared keys or certificates, satisfying the requirement for encrypting all traffic and authenticating both data centers.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
SSH
Why it's wrong here
SSH is for secure terminal access, not site-to-site.
IPsec tunnel mode
Why this is correct
IPsec tunnel mode encrypts and authenticates entire packets between gateways.
Related concept
Read the scenario before looking for a memorised answer.
MPLS
Why it's wrong here
MPLS provides traffic engineering but no encryption.
SSL/TLS
Common exam traps
ISC2 often tests the distinction between IPsec tunnel mode and transport mode, and candidates may confuse SSL/TLS (which secures individual sessions) with a full network-layer VPN solution, missing that IPsec tunnel mode is the only option that encrypts all traffic and authenticates both endpoints at the network layer.
Detailed technical explanation
IPsec tunnel mode uses ESP (Encapsulating Security Payload) to encrypt the entire original IP packet and appends a new IP header, with the source and destination addresses of the VPN gateways. The IKEv2 protocol (RFC 7296) establishes a secure association using Diffie-Hellman key exchange and mutual authentication, often via X.509 certificates, ensuring both endpoints are verified before any data is transmitted. In a real-world scenario, this is commonly deployed for connecting AWS VPCs to on-premises data centers via VPN, where the tunnel mode protects all traffic (including routing protocols like BGP) across the public internet.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
What to study next
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Communication and Network Security — This question tests Communication and Network Security — Read the scenario before looking for a memorised answer..
The correct answer is: IPsec tunnel mode — IPsec tunnel mode is the correct choice because it encrypts the entire IP packet, including the original IP header, and encapsulates it within a new IP header for secure transport across an untrusted WAN. It also provides mutual authentication of both endpoints using IKE (Internet Key Exchange) with pre-shared keys or certificates, satisfying the requirement for encrypting all traffic and authenticating both data centers.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Same concept, more angles
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Variation 1. A security architect is designing a secure communication channel between two remote sites over the internet. Which TWO of the following protocols should be used to ensure confidentiality, integrity, and authentication?
mediumWhy C: Options B (IPsec with ESP in tunnel mode) and E (L2TP over IPsec) provide encryption and authentication for site-to-site VPNs. PPTP is outdated and insecure. SSL/TLS is primarily used for client-to-site VPNs. MPLS does not provide encryption.
Last reviewed: Jun 30, 2026
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.
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