VPN standards do exist, but they cannot overcome all the interoperability issues. The IPSec VPN standard was adopted several years ago and has been implemented in most commercial VPN products. But to set their products apart, many VPN vendors have added proprietary functionality that does not work in multivendor environments and can make configuration more difficult.

Because the IPSec standard does not include support for user authentication, for example, vendors typically add proprietary extensions to improve user authentication for remote-access connections. Vendors also often add extensions to provide support for NAT (Network Address Translation).

To achieve interoperability in multivendor environments, users should avoid the use of VPN functions that fall outside the IPSec standard. Interoperability will also require a solid understanding of IPSec. Sticking to a basic VPN connection will allow a company to create a secure connection among VPN products from different vendors, but preclude the use of all of the offered bells and whistles.

There are two main types of VPN connections: site-to-site connections using VPN gateways and remote access connections using VPN client software. Each connection type has its own interoperability issues.

Site-to-site connections are configured on each IPSec gateway by defining two endpoints (or subnets),