The term VLAN subinterface correctly implies the VLAN interface is not a complete interface by itself. You add a VLAN subinterface to the physical interface that receives VLAN-tagged packets. The physical interface can belong to a different VDOM than the VLAN, but it must be connected to a network route that is configured for this VLAN. Without that route, the VLAN will not be connected to the network, and VLAN traffic will not be able to access this interface.The traffic on the VLAN is separate from any other traffic on the physical interface.
FortiGate unit interfaces cannot have overlapping IP addresses, the IP addresses of all interfaces must be on different subnets. This rule applies to both physical interfaces and to virtual interfaces such as VLAN subinterfaces. Each VLAN subinterface must be configured with its own IP address and netmask. This rule helps prevent a broadcast storm or other similar network problems.
Any FortiGate unit, with or without VDOMs enabled, can have a maximum of 255 interfaces in Transparent operating mode. In NAT/Route operating mode, the number can range from 255 to 8192 interfaces per VDOM, depending on the FortiGate model. These numbers include VLANs, other virtual interfaces, and physical interfaces. To have more than 255 interfaces configured in Transparent operating mode, you need to configure multiple VDOMs with many interfaces on each VDOM.
This example shows how to add a VLAN, vlan_accounting on the FortiGate unit internal interface with an IP address of 10.13.101.101.
To add a VLAN – web-based manager
1. Go to System > Network > Interface and select Create New.
The Type is by default set to VLAN.
2. Enter a name for the VLAN to vlan_accounting.
3. Select the Internal interface.
4. Enter the VLAN ID.
The VLAN ID is a number between 1 and 4094 that allow groups of IP addresses with the same VLAN ID to be associated together.
5. Select the Addressing Mode of Manual.
6. Enter the IP address for the port of 10.13.101.101/24.
7. Set the Administrative Access to HTTPS and SSH.
8. Select OK.
To add a VLAN – CLI
config system interface edit VLAN_1
set interface internal set type vlan
set vlanid 100
set ip 10.13.101.101/24 set allowaccess https ssh
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