IPsec VPN overview

IPsec VPN overview

This section provides a brief overview of IPsec technology and includes general information about how to configure IPsec VPNs using this guide.

The following topics are included in this section: Types of VPNs

Planning your VPN General preparation steps

How to use this guide to configure an IPsec VPN

VPN configurations interact with the firewall component of the FortiGate unit. There must be a security policy in place to permit traffic to pass between the private network and the VPN tunnel.

Security policies for VPNs specify:

  • The FortiGate interface that provides the physical connection to the remote VPN gateway, usually an interface connected to the Internet
  • The FortiGate interface that connects to the private network
  • IP addresses associated with data that has to be encrypted and decrypted
  • Optionally, a schedule that restricts when the VPN can operate
  • Optionally, the services (types of data) that can be sent

When the first packet of data that meets all of the conditions of the security policy arrives at the FortiGate unit, a VPN tunnel may be initiated and the encryption or decryption of data is performed automatically afterward. For more information, see Defining VPN security policies on page 1648.

Where possible, you should create route-based VPNs. Generally, route-based VPNs are more flexible and easier to configure than policy-based VPNs — by default they are treated as interfaces. However, these two VPN types have different requirements that limit where they can be used.


Types of VPNs

FortiGate unit VPNs can be policy-based or route-based. There is little difference between the two types. In both cases, you specify Phase 1 and Phase 2 settings. However there is a difference in implementation. A route-based VPN creates a virtual IPsec network interface that applies encryption or decryption as needed to any traffic that it carries. That is why route-based VPNs are also known as interface-based VPNs. A policy-based VPN is implemented through a special security policy that applies the encryption you specified in the Phase 1 and Phase 2 settings.


Routebased VPNs

For a route-based VPN, you create two security policies between the virtual IPsec interface and the interface that connects to the private network. In one policy, the virtual interface is the source. In the other policy, the virtual interface is the destination. This creates bidirectional policies that ensure traffic will flow in both directions over the VPN.

A route-based VPN is also known as an interface-based VPN.

Each route-based IPsec VPN tunnel requires a virtual IPsec interface. As such, the amount of possible route-based IPsec VPNs is limited by the system.interface table size. The system.interface table size for most devices is 8192.

For a complete list of table sizes for all devices, refer to the Maximum Values table.


Policybased VPNs

For a policy-based VPN, one security policy enables communication in both directions. You enable inbound and outbound traffic as needed within that policy, or create multiple policies of this type to handle different types of traffic differently. For example HTTPS traffic may not require the same level of scanning as FTP traffic.

A policy-based VPN is also known as a tunnel-mode VPN.


Comparing policy-based or route-based VPNs

For both VPN types you create Phase 1 and Phase 2 configurations. Both types are handled in the stateful inspection security layer, assuming there is no IPS or AV. For more information on the three security layers, see the FortiOS Troubleshooting guide.

The main difference is in the security policy.

You create a policy-based VPN by defining an IPSEC security policy between two network interfaces and associating it with the VPN tunnel (Phase 1) configuration.

You create a route-based VPN by creating a virtual IPsec interface. You then define a regular ACCEPT security policy to permit traffic to flow between the virtual IPsec interface and another network interface. And lastly, configure a static route to allow traffic over the VPN.

Where possible, you should create route-based VPNs. Generally, route-based VPNs are more flexible and easier to configure than policy-based VPNs — by default they are treated as interfaces. However, these two VPN types have different requirements that limit where they can be used.


Comparison of policy-based and route-based VPNs


Features Policy-based Route-based

Both NAT and transparent modes available




NAT mode only


L2TPoverIPsec supported






GREoverIPsec supported







security policy requirements


Requires a security policy with IPSEC action that specifies the VPN tunnel


Requires only a simple security policy with ACCEPT action


Number of policies per VPN


One policy controls connections in both directions


A separate policy is required for connections in each direction



Planning your VPN

It is a good idea to plan the VPN configuration ahead of time. This will save time later and help you configure your VPN correctly.

All VPN configurations are comprised of numerous required and optional parameters. Before you begin, you need to determine:

  • Where the IP traffic originates and where it needs to be delivered
  • Which hosts, servers, or networks to include in the VPN
  • Which VPN devices to include in the configuration
  • Through which interfaces the VPN devices communicate
  • Through which interfaces do private networks access the VPN gateways

Once you have this information, you can select a VPN topology that suits the network environment.


Network topologies

The topology of your network will determine how remote peers and clients connect to the VPN and how VPN traffic is routed.


VPN network topologies and brief descriptions

Topology                                 Description

Gateway-to-gateway con- figurations

Standard one-to-one VPN between two FortiGate units. See Gateway-to- gateway configurations on page 1655.

Hub-and-spoke configurations     One central FortiGate unit has multiple VPNs to other remote FortiGate units. See Hub-and-spoke configurations on page 1671.

Dynamic DNS configuration        One end of the VPN tunnel has a changing IP address and the other end must go to a dynamic DNS server for the current IP address before estab- lishing a tunnel. See Dynamic DNS configuration on page 1688.

Typically remote FortiClient dialup-clients use dynamic IP addresses through NAT devices. The FortiGate unit acts as a dialup server allowing dialup VPN connections from multiple sources. See FortiClient dialup-client configurations on page 1702.

Similar to FortiClient dialup-client configurations but with more gateway-to- gateway settings such as unique user authentication for multiple users on a single VPN tunnel. See FortiGate dialup-client configurations on page 1716.

Secure web browsing performed by dialup VPN clients, and/or hosts behind a remote VPN peer. See Internet-browsing configuration on page 1729.


Topology                                 Description

Redundant VPN con- figurations

Options for supporting redundant and partially redundant IPsec VPNs, using route-based approaches. See Redundant VPN configurations on page 1734.

Transparent mode VPNs

In transparent mode, the FortiGate acts as a bridge with all incoming traffic being broadcast back out on all other interfaces. Routing and NAT must be performed on external routers. See Transparent mode VPNs on page 1759.

L2TP and IPsec (Microsoft VPN)

Configure VPN for Microsoft Windows dialup clients using the built in L2TP software. Users do not have to install any See L2TP and IPsec (Microsoft VPN) on page 1778.

These sections contain high-level configuration guidelines with cross-references to detailed configuration procedures. If you need more detail to complete a step, select the cross-reference in the step to drill-down to more detail. Return to the original procedure to complete the procedure. For a general overview of how to configure a VPN, see Planning your VPN .


General preparation steps

A VPN configuration defines relationships between the VPN devices and the private hosts, servers, or networks making up the VPN. Configuring a VPN involves gathering and recording the following information. You will need this information to configure the VPN.

  • The private IP addresses of participating hosts, servers, and/or networks. These IP addresses represent the source addresses of traffic that is permitted to pass through the VPN. A IP source address can be an individual IP address, an address range, or a subnet address.
  • The public IP addresses of the VPN end-point interfaces. The VPN devices establish tunnels with each other through these interfaces.
  • The private IP addresses associated with the VPN-device interfaces to the private networks. Computers on the private networks behind the VPN gateways will connect to their VPN gateways through these interfaces.

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About Mike

Michael Pruett, CISSP has a wide range of cyber-security and network engineering expertise. The plethora of vendors that resell hardware but have zero engineering knowledge resulting in the wrong hardware or configuration being deployed is a major pet peeve of Michael's. This site was started in an effort to spread information while providing the option of quality consulting services at a much lower price than Fortinet Professional Services. Owns PacketLlama.Com (Fortinet Hardware Sales) and Office Of The CISO, LLC (Cybersecurity consulting firm).

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