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Difficulty: HardVirtual Private Networks and Remote Access Security

Engineers troubleshooting a remote access IPsec IKEv2 VPN deployment notice that clients can successfully complete initial IKE negotiations over UDP port 500 when assigned a public IPv4 address directly. However, when telecommuters attempt to connect from behind residential routers performing Port Address Translation (PAT), Phase 2 negotiation fails and no encrypted data passes through the tunnel because the intermediate PAT router drops IP Protocol 50 (ESP) packets. Which configuration modification on the VPN gateway will resolve this connection issue while preserving payload encryption for telecommuters?

  1. Enable NAT Traversal (NAT-T) on the gateway to encapsulate IPsec ESP packets inside UDP port 4500 datagrams.Answer
  2. B
    Switch the IPsec security protocol from Encapsulating Security Payload (ESP) to Authentication Header (AH) operating in transport mode.
  3. C
    Reconfigure the IKE negotiation parameters on the gateway to transmit key exchange traffic over TCP port 500 instead of UDP port 500.
  4. D
    Rebind the IPsec VPN service port to listen for raw ESP packet encapsulation over TCP port 443.

Answer

Enable NAT Traversal (NAT-T) on the gateway to encapsulate IPsec ESP packets inside UDP port 4500 datagrams.
Enabling NAT Traversal (NAT-T) dynamically detects intermediate NAT/PAT devices during IKE negotiations. Upon detection, the gateway and client encapsulate IPsec ESP packets (IP Protocol 50) within UDP datagrams using destination port 4500. Because UDP port 4500 includes standard Layer 4 port headers, intermediate routers performing Port Address Translation (PAT) can translate and forward the traffic without dropping the VPN connection or invalidating payload encryption.

Step-by-Step Solution

1
Analyze the protocol failure cause in the scenario.
IPsec Encapsulating Security Payload (ESP) operates directly over IP as Protocol 50 without Layer 4 TCP or UDP port headers.
Because Port Address Translation (PAT) relies on modifying Layer 4 TCP/UDP port numbers, intermediate PAT routers frequently drop or fail to map raw IP Protocol 50 ESP traffic.
2
Evaluate standard IPsec solutions for PAT compatibility.
NAT Traversal (NAT-T) is defined in RFC standards (RFC 3947/3948) specifically to detect NAT/PAT devices during IKE Phase 1.
When NAT is detected, NAT-T wraps the ESP packet inside a standard UDP header using destination port 4500.
3
Verify that payload security and data path function correctly.
Intermediate PAT routers can successfully track and translate UDP port 4500 sessions, ensuring encrypted VPN payloads pass through while maintaining full ESP encryption integrity.
Encapsulating ESP in UDP port 4500 allows PAT translation without breaking the cryptographic integrity of the original payload.

Key Concept

IPsec NAT Traversal (NAT-T) and UDP Port 4500 Encapsulation
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Topics

Virtual Private Networks and Remote Access Security

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