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ZyWALL (ZLD) CLI Reference Guide 141
CHAPTER 17
IPSec VPN
This chapter explains how to set up and maintain IPSec VPNs in the ZyWALL.
17.1 IPSec VPN Overview
A virtual private network (VPN) provides secure communications between sites without the expense
of leased site-to-site lines. A secure VPN is a combination of tunneling, encryption, authentication,
access control and auditing. It is used to transport traffic over the Internet or any insecure network
that uses TCP/IP for communication.
Internet Protocol Security (IPSec) is a standards-based VPN that offers flexible solutions for secure
data communications across a public network like the Internet. IPSec is built around a number of
standardized cryptographic techniques to provide confidentiality, data integrity and authentication
at the IP layer.
The following figure is one example of a VPN tunnel.
Figure 19 VPN: Example
The VPN tunnel connects the ZyWALL (X) and the remote IPSec router (Y). These routers then
connect the local network (A) and remote network (B).
A VPN tunnel is usually established in two phases. Each phase establishes a security association
(SA), a contract indicating what security parameters the ZyWALL and the remote IPSec router will
use. The first phase establishes an Internet Key Exchange (IKE) SA between the ZyWALL and
remote IPSec router. The second phase uses the IKE SA to securely establish an IPSec SA through