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Difficulty: HardVirtual Private Networks and Remote Access Security

Remote workers attempting to connect to an enterprise network using an IPsec IKEv2 remote access client report that authentication succeeds, but no internal network resources are accessible and tunnel data transmission fails immediately after connection setup. The edge network security monitoring tool shows that Phase 1 (IKE) negotiations complete successfully over UDP port 500, but Encapsulating Security Payload (ESP) packets sent during Phase 2 are silently dropped by the remote users' Small Office/Home Office (SOHO) routers performing Port Address Translation (PAT). Which of the following configuration changes will resolve this connectivity issue while maintaining secure, encrypted communications?

  1. Enable NAT Traversal (NAT-T) on the VPN headend gateway to encapsulate ESP data within UDP port 4500 packets.Answer
  2. B
    Switch the IPsec protocol configuration from Encapsulating Security Payload (ESP) mode to Authentication Header (AH) mode.
  3. C
    Configure the VPN gateway to switch Phase 2 payload encryption transport from UDP port 500 to TCP port 500.
  4. D
    Deploy Generic Routing Encapsulation (GRE) natively without IPsec to eliminate protocol encapsulation overhead.

Answer

Enabling NAT Traversal (NAT-T) on the VPN headend gateway encapsulates ESP traffic inside UDP port 4500 packets, allowing the IPsec tunnel to traverse PAT/NAT devices without dropping Phase 2 traffic.
Standard IPsec Encapsulating Security Payload (ESP) operates at Layer 3 using IP protocol number 50. Because ESP lacks Layer 4 TCP/UDP port headers, SOHO routers using Port Address Translation (PAT) frequently drop or misroute raw ESP packets. Enabling NAT Traversal (NAT-T) encapsulates ESP packets inside standard UDP datagrams targeting port 4500, permitting PAT routers to properly process and forward the encrypted tunnel traffic.

Step-by-Step Solution

1
Analyze the reported issue and network path symptoms.
Phase 1 (IKE) succeeds over UDP port 500, but Phase 2 data transport fails when raw ESP traffic (IP protocol 50) encounters intermediate SOHO routers performing Port Address Translation (PAT).
PAT devices rebind Layer 4 TCP/UDP ports, but standard ESP operates directly on Layer 3 (IP protocol 50) without TCP or UDP headers, causing many basic consumer NAT routers to drop or fail to track ESP sessions correctly.
2
Evaluate potential solutions for IPsec NAT compatibility.
NAT Traversal (NAT-T) automatically detects intermediate NAT/PAT devices during Phase 1 negotiations (IKE) and wraps the raw ESP packets inside UDP headers using port 4500.
Because UDP port 4500 contains standard Layer 4 port headers, PAT gateways can cleanly translate and track the stateful VPN session.
3
Verify protocol enforcement and security posture.
Enabling NAT-T on the headend gateway allows remote users behind home PAT routers to establish a fully encrypted IPsec tunnel without sacrificing confidentiality.
NAT-T maintains ESP encryption while resolving the NAT handling conflict.

Key Concept

IPsec NAT Traversal (NAT-T) and UDP Port 4500 Encapsulation
Estimated Time:2m 0s
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Topics

Virtual Private Networks and Remote Access Security

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