Inter-VDOM routing

InterVDOM routing

Inter-VDOM routing changes this allows VDOMs to communicate internally without using additional physical interfaces, using VDOM links. VDOM links are virtual interfaces that connect VDOMs. A VDOM link contains a pair of interfaces with each one connected to a VDOM, and forming either end of the inter-VDOM connection.


This chapter contains the following sections:

  • Benefits of inter-VDOM routing
  • Configuring VDOM links
  • Inter-VDOM configurations
  • Dynamic routing over inter-VDOM links
  • HA virtual clusters and VDOM links
  • Example configuration: Inter-VDOM routing


Benefits of inter-VDOM routing

Inter-VDOM routing has a number of advantages over independent VDOM routing. These benefits include:

  • Freed-up physical interfaces
  • More speed than physical interfaces
  • Continued support for secure firewall policies
  • Configuration flexibility


Freedup physical interfaces

Tying up physical interfaces on the FortiGate unit presents a problem. With a limited number of interfaces available, configuration options for the old style of communication between VDOMs are very limited. VLANs can be an answer to this, but they have some limitations.

For example, the FortiGate-800 has 8 physical ethernet ports. If they are assigned 2 per VDOM (one each for external and internal traffic) there can only be 4 VDOMs at most configured, not the 10 VDOMs the license will allow. Adding even one additional interface per VDOM to be used to communicate between VDOMs leaves only 2 VDOMs for that configuration, since it would required 9 interfaces for 3 VDOMs. Even using one physical interface for both external traffic and inter-VDOM communication would severely lower the available bandwidth for external traffic on that interface.

With the introduction of inter-VDOM routing, traffic can travel between VDOMs internally, freeing up physical interfaces for external traffic. Using the above example we can use the 4 VDOM configuration and all the interfaces will have their full bandwidth.


More speed than physical interfaces

Internal interfaces are faster than physical interfaces. Their speed depends on the FortiGate unit CPU and its load. That means that an inter-VDOM link interface will be faster than a outbound physical interface connected to another inbound physical interface.

Inter-VDOM links are CPU bound, and cannot be part of an accelerated pair of interfaces.

However, while one virtual interface with normal traffic would be considerably faster than on a physical interface, the more traffic and more internal interfaces you configure, the slower they will become until they are slower than the physical interfaces. CPU load can come from other sources such as AV or content scanning. This produces the same effect—internal interfaces such as inter-VDOM links will be slower.


Continued support for secure firewall policies

VDOMs help to separate traffic based on your needs. This is an important step in satisfying regulations that require proof of secure data handling. This is especially important to health, law, accounting, and other businesses that handle sensitive data every day.

By keeping things separate, traffic has to leave the FortiGate unit and re-enter to change VDOMs. This forces traffic to go through the firewall when leaving and enter through another firewall, keeping traffic secure.

With inter-VDOM routing, the need for the physical interfaces is greatly reduced. However, firewall policies still need to be in place for traffic to pass through any interface, physical or virtual, and thus provide the same level of security both internally and externally. Configuration of firewall policies is the same for inter-VDOM links as for any other interface, and your data will continue to have the high level of security.


Configuration flexibility

A typical VDOM uses at least two interfaces, typically physical interfaces, one for internal and one for external traffic. Depending on the configuration, more interfaces may be required. This means that the maximum number of VDOMs configurable on a FortiGate unit using physical interfaces is the number of interfaces available divided by two. VLANs can increase the number by providing multiple virtual interfaces over a single physical interface, but VLANs have some limitations. Using physical interfaces for inter-VDOM communication therefore limits the number of possible configurations on your FortiGate unit.

To overcome this limitation, inter-VDOM links can be created within the FortiGate unit. Using virtual interfaces, inter-VDOM links free up the physical interfaces for external traffic. Using VDOM links on a FortiGate unit with 8 physical interfaces, you can have 4 VDOMs communicating with each other (meshed configuration) and continue to have 2 physical interfaces each for internal and external connections. This configuration would have required 20 physical interfaces without inter-VDOM routing. With inter-VDOM routing it only requires 8 physical interfaces, with the other 12 interfaces being internal VDOM links.

Inter-VDOM routing allows you to make use of standalone VDOMs, Management VDOMs, and Meshed VDOMs without being limited by the number of physical interfaces on your FortiGate unit. For more information about these types of VDOMs, see “Inter-VDOM configurations” on page 2639.

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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).

2 thoughts on “Inter-VDOM routing

  1. René Jorissen

    Would it be useful to mention that inter-VDOM links are only possible when the VDOMs are operational on the same physical firewall?

    Inter-VDOM links don’t work when VDOM partitioning is used to share load over 2 physical firewalls.

    1. Mike Post author

      Good point. I have never seen someone attempt to span it across two physical firewalls….At least not without it being an HA pair or something of that sort (granted logically it is still treated as though it is a single firewall in that case)


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