Figure 228 Vpn/Nat Example - ZyXEL Communications 5 Series User Manual

Chapter 19 IPSec VPN
VPN, NAT, and NAT Traversal
In the following example, there is another router (A) between router X and router Y.

Figure 228 VPN/NAT Example

If router A does NAT, it might change the IP addresses, port numbers, or both. If router X and
router Y try to establish a VPN tunnel, the authentication fails because it depends on this
information. The routers cannot establish a VPN tunnel.
Most routers like router A now have an IPSec pass-through feature. This feature helps router A
recognize VPN packets and route them appropriately. If router A has this feature, router X and
router Y can establish a VPN tunnel as long as the active protocol is ESP. (See
on page 394
If router A does not have an IPSec pass-through or if the active protocol is AH, you can solve
this problem by enabling NAT traversal. In NAT traversal, router X and router Y add an extra
header to the IKE SA and IPSec SA packets. If you configure router A to forward these
packets unchanged, router X and router Y can establish a VPN tunnel.
You have to do the following things to set up NAT traversal.
• Enable NAT traversal on the ZyWALL and remote IPSec router.
• Configure the NAT router to forward packets with the extra header unchanged. (See the
field description for detailed information about the extra header.)
The extra header may be UDP port 500 or UDP port 4500, depending on the standard(s) the
ZyWALL and remote IPSec router support.
IPSec SA Overview
Once the ZyWALL and remote IPSec router have established the IKE SA, they can securely
negotiate an IPSec SA through which to send data between computers on the networks.
The IPSec SA stays connected even if the underlying IKE SA is not available
anymore.
This section introduces the key components of an IPSec SA.
Local and Remote Networks
In an IPSec SA, the local network consists of devices connected to the ZyWALL and may be
called the local policy. Similarly, the remote network consists of the devices connected to the
remote IPSec router and may be called the remote policy.
392
for more information about active protocols.)
Active Protocol
ZyWALL 5/35/70 Series User's Guide