ZyXEL Communications EMG5324-D10A User Manual

 Chapter 17 VPN

EMG5324-D10A User’s Guide

• Use ESP security protocol (in either transport or tunnel mode).
• Use  IKE  keying  mode.
• Enable NAT traversal on both IPSec endpoints.
• Set the NAT router to forward UDP port 500 to IPSec router A.

Finally, NAT is compatible with ESP in tunnel mode because integrity checks are performed over the 
combination of the "original header plus original payload," which is unchanged by a NAT device. The 
compatibility of AH and ESP with NAT in tunnel and transport modes is summarized in the following 
table.

Y* - This is supported in the Device if you enable NAT traversal.

The two modes of operation for IPSec VPNs are Transport mode and Tunnel mode. 

Transport and Tunnel Mode IPSec Encapsulation

Tunnel mode encapsulates the entire IP packet to transmit it securely. A Tunnel mode is required 
for gateway services to provide access to internal systems. Tunnel mode is fundamentally an IP 
tunnel with authentication and encryption. This is the most common mode of operation. Tunnel 
mode is required for gateway to gateway and host to gateway communications. Tunnel mode 
communications have two sets of IP headers:

• Outside header: The outside IP header contains the destination IP address of the VPN gateway.
• Inside header: The inside IP header contains the destination IP address of the final system 

behind the VPN gateway. The security protocol appears after the outer IP header and before the 
inside IP header. 

VPN and NAT

AH

Transport

N

AH

Tunnel

N

ESP

Transport

Y*

ESP

Tunnel

Y