mediumMultiple ChoiceObjective-mapped
CISM NAT Traversal (NAT-T) Practice Question
Exhibit
Exhibit: ! crypto isakmp policy 10 encr aes authentication pre-share group 2 lifetime 86400 ! crypto ipsec transform-set TS esp-aes esp-sha-hmac ! crypto map VPN 10 ipsec-isakmp set peer 10.1.1.1 set transform-set TS match address 101 ! interface FastEthernet0/0 crypto map VPN !
A security administrator reports that the VPN tunnel to the remote peer (10.1.1.1) intermittently fails. Which of the following is the most likely cause?
⚠ Common exam trap
A common pitfall is to assume that intermittent VPN failures are due to IKE phase mismatches or ACL errors, but NAT traversal issues often manifest as intermittent rather than full failures.
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
✓
Improper NAT traversal configuration
The intermittent failure suggests NAT traversal is not properly configured. NAT traversal (NAT-T) is used to encapsulate VPN traffic (ESP) in UDP to traverse NAT devices. If the remote peer is behind a NAT device and NAT-T is not correctly enabled or configured, packets may be dropped, causing intermittent tunnel failures.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Improper NAT traversal configuration
Why this is correct
NAT traversal (NAT-T) is needed when a VPN endpoint is behind a NAT device. Improper configuration can cause intermittent failures as packets may be dropped during translation.
- ✗
Mismatched IKE phase 1 parameters
Why it's wrong here
Mismatched IKE phase 1 parameters typically cause tunnel negotiation to fail completely, not intermittently.
- ✗
Expired digital certificate
Why it's wrong here
An expired digital certificate would cause authentication failure and prevent the tunnel from establishing at all, not intermittent failures.
- ✗
Incorrect firewall access control list
Why it's wrong here
An incorrect ACL would either block all traffic or allow it consistently; it would not typically cause intermittent behavior.
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.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This CISM practice question is part of Courseiva's free ISACA 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 CISM exam.