Multicast forwarding and FortiGate units

Multicast forwarding and FortiGate units

In both transparent mode and NAT mode you can configure FortiGate units to forward multicast traffic.

For a FortiGate unit to forward multicast traffic you must add FortiGate multicast security policies. Basic multicast security policies accept any multicast packets at one FortiGate interface and forward the packets out another FortiGate interface. You can also use multicast security policies to be selective about the multicast traffic that is accepted based on source and destination address, and to perform NAT on multicast packets.

In the example shown below, a multicast source on the Marketing network with IP address sends multicast packets to the members of network At the FortiGate unit, the source IP address for multicast packets originating from workstation is translated to In this example, the FortiGate unit is not acting as a multicast router.


Multicast forwarding and RIPv2

RIPv2 uses multicast to share routing table information. If your FortiGate unit is installed on a network that includes RIPv2 routers, you must configure the FortiGate unit to forward multicast packets so that RIPv2 devices can share routing data through the FortiGate unit. No special FortiGate configuration is required to share RIPv2 data, you can simply use the information in the following sections to configure the FortiGate unit to forward multicast packets.

RIPv1 uses broadcasting to share routing table information. To allow RIPv1 packets through a FortiGate unit you can add standard security policies. Security policies to accept RIPv1 packets can use the ANY predefined firewall service or the RIP pre- defined firewall service.


Example multicast network including a FortiGate unit that forwards multicast packets



Configuring FortiGate multicast forwarding

You configure FortiGate multicast forwarding from the Command Line Interface (CLI). Two steps are required:

  • Adding multicast security policies
  • Enabling multicast forwarding

This second step is only required if your FortiGate unit is operating in NAT mode. If your FortiGate unit is operating in transparent mode, adding a multicast policy enables multicast forwarding.

There is sometimes a confusion between the terms “forwarding” and “routing”. These two functions should not be taking place at the same time.

It is mentioned that multicast-forward should be enabled when the FortiGate unit is in NAT mode and that this will forward any multicast packet to all interfaces. However, this parameter should NOT be enabled when the FortiGate unit operates as a mul- ticast router (i.e. with a routing protocol enabled. It should only be enabled when there is no routing protocols activated.


Adding multicast security policies

You need to add security policies to allow packets to pass from one interface to another. Multicast packets require multicast security policies. You add multicast security policies from the CLI using the config firewall multicast-policy command. As with unicast security policies, you specify the source and destination interfaces and optionally the allowed address ranges for the source and destination addresses of the packets.


You can also use multicast security policies to configure source NAT and destination NAT for multicast packets. Keep the following in mind when configuring multicast security policies:

  • The matched forwarded (outgoing) IP multicast source IP address is changed to the configured IP address.
  • Source and Destination interfaces are optional. If left blank, then the multicast will be forwarded to ALL interfaces.
  • Source and Destination addresses are optional. If left un set, then it will mean ALL addresses.
  • The nat keyword is optional. Use it when source address translation is needed.
This entry was posted in Fortinet GURU, FortiOS 5.4 Handbook on by .

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).

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.