VLANs vs forwarding domains

There are several differences between VLAN and a forwarding domain configured on a FortiGate in transparent mode:

• A forwarding domain is used to create separated broadcast domains between VLANs and allow independent VLAN learning - IVL (MAC addresses in the FDB). This would be equivalent to creating VLANs on a regular L2 switch.

When VLANs are used in the network, configuring different forwarding domains is essential to avoid broadcast duplications. See also section Default VLAN forwarding behavior for additional information.

• VLANs configured on interfaces are only used for tagging packets egressing the port and classifying packets at ingress.
• The packets processed by the direct interface (or port) itself are always sent untagged and must be received untagged.