Every single interface on the firewall is separated by using VLANs internally. By default, it starts at 2. If you are configuring VLAN sub-interfaces or using VLANs on the switch directly connected to the firewall using the same VLANs might cause unexpected issues.
From firmware version 6.5.3.x onwards, we can now configure the internal VLAN so that they do not overlap with other VLANs connected/configured on the firewall.
Enter the VLAN ID in the Starting VLAN ID field. The default ID is 2.
For convenience, a link to the MANAGE I Security Configuration > Advanced Settings page has been added to the MANAGE I System Setup > Switching > VLAN Trunking page.
Also, you can only select the first VLAN number(starting VLAN ID). It assigns that VLAN to the first interface i.e., X0 and keeps incrementing it by 1 for all subsequent interfaces. We cannot assign VLANs to each of the interfaces separately.
NOTE: For the configured internal VLAN to take effect, you must restart the firewall. When you configure the internal VLAN, a RESTART button appears on the MANAGE I Security Configuration | Firewall Settings | Advanced Settings page.