Configuring multicast forwarding

There is sometimes confusion between the terms forwarding and routing. These two functions should not take place at the same time. Multicast forwarding should be enabled when the FortiGate is in NAT mode and you want to forward multicast packets between multicast routers and receivers. However, this function should not be enabled when the FortiGate itself is operating as a multicast router, or has an applicable routing protocol that uses multicast.

Multicast forwarding is not supported on enhanced MAC VLAN interfaces. To use multicast with enhanced MAC VLAN interfaces, use PIM (Multicast routing and PIM support).

There are two steps to configure multicast forwarding:

  1. Enabling multicast forwarding
  2. Configuring multicast policies

Enabling multicast forwarding

Multicast forwarding is enabled by default. If a FortiGate is operating in transparent mode, adding a multicast policy enables multicast forwarding. In NAT mode you must use the multicast-forward setting to enable or disable multicast forwarding.

Multicast forwarding in NAT mode

When multicast-forward is enabled, the FortiGate forwards any multicast IP packets in which the TTL is 2 or higher to all interfaces and VLAN interfaces, except the receiving interface. The TTL in the IP header will be reduced by 1. Even though the multicast packets are forwarded to all interfaces, you must add multicast policies to allow multicast packets through the FortiGate.

To enable multicast forwarding in NAT mode:
config system settings
    set multicast-forward enable

Prevent the TTL for forwarded packets fro