156-215.81.20 VPN Basics Practice Question
What is the purpose of 'Anti-Replay' in an IPsec VPN?
⚠ Common exam trap
Candidates frequently mistake anti-replay mechanisms for encryption mechanisms that guarantee data confidentiality, confusing packet sequencing validation with payload secrecy.
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
✓
To prevent unauthorized packet injection
Anti-Replay prevents an attacker from capturing encrypted packets and re-transmitting them later to force the gateway to process them again. This is critical because, even if the attacker cannot decrypt the packets, replaying them might cause an application error or lead to unauthorized actions if the system does not verify the uniqueness of each packet. It is a standard safety feature for all modern IPsec VPNs.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
To speed up the encryption process
Why it's wrong here
Anti-Replay is a security feature, not a performance optimization. It actually requires a small amount of overhead to keep track of packet sequence numbers, which is necessary to ensure each packet is unique and processed exactly once by the receiver.
- ✓
To prevent unauthorized packet injection
Why this is correct
Anti-Replay ensures that every packet has a unique sequence number. If a gateway receives a packet with a duplicate number or an out-of-order packet that falls outside the allowed window, it drops it, preventing attackers from injecting old, valid packets into the current stream.
- ✗
To compress data before encryption
Why it's wrong here
Compression is a feature designed to reduce the size of the payload for better bandwidth efficiency. Anti-Replay is entirely separate and deals with the security of the transport layer, specifically focusing on the integrity and uniqueness of each packet in the stream.
- ✗
To manage the VPN tunnel's timeout
Why it's wrong here
Tunnel timeouts are controlled by the IKE Phase 1 and Phase 2 lifetimes. Anti-Replay is a packet-level mechanism and has no role in determining how long a tunnel remains active or when a re-keying process should be triggered by the gateway.
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.
About these practice questions
This 156-215.81.20 question is part of Courseiva's 210-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Check Point exam blueprint
This 156-215.81.20 practice question is part of Courseiva's free Check Point 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 156-215.81.20 exam.