Technical Tip: Unable to add an additional link to FortiLink

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Description	This article describes an issue when adding an interface (port) to an existing FortiLink aggregate link gives an error.
Scope	FortiGate.
Solution	In a production environment where FortiLink is configured as an aggregate link and required to add another link due to high-capacity requirements, an error may occur as below. This is because a VLAN under the FortiLink is created with 802.1ad, which is called a Q-in-Q VLAN. It is possible to add a new link/interface to the FortiLink, only by deleting the VLAN interface. Note: Once the VLAN has been created, neither VLAN ID, VLAN protocol, nor physical interface can be changed. Note: VLANs with 802.1q configuration do not have this issue The main root cause is that FortiLink is fundamentally designed to manage and forward single-tag 802.1Q VLANs, where each VLAN directly maps. When 802.1ad (Q-in-Q) is used, traffic carries nested VLAN tags, but FortiLink and FortiSwitch hardware primarily recognize and control only the outer VLAN tag. This breaks the internal VLAN-to-interface model, causes inner VLANs to be treated inconsistently, and often disables hardware offloading. As a result, multiple Q-in-Q VLANs do not scale or behave correctly under FortiLink, while standard 802.1Q VLANs work reliably because they match the architecture FortiLink was built to support.