A VLAN subinterface, also called a VLAN, is a virtual interface on a physical interface. The subinterface allows routing of VLAN tagged packets using that physical interface, but it is separate from any other traffic on the physical interface.
Adding a VLAN subinterface includes configuring: • Physical interface
• IP address and netmask • VLAN ID
• VDOM
Physical interface
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 router that is configured for this VLAN. Without that router, 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.
When you are working with interfaces on your FortiGate unit, use the Column Settings on the Interface display to make sure the information you need is displayed. When working with VLANs, it is useful to position the VLAN ID column close to the IP address. If you are working with VDOMs, including the Virtual Domain column as well will help you troubleshoot problems more quickly.
VLANs Page 161 Install and System Administration for FortiOS 5.0 IP address and netmask
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 pair. This rule helps prevent a broadcast storm or other similar network problems.
If you are unable to change your existing configurations to prevent IP overlap, enter the CLI command config system global and set ip-overlap enable to allow IP address overlap. If you enter this command, multiple VLAN interfaces can have an IP address that is part of a subnet used by another interface. This command is recommended for advanced users only.
VLAN ID
The VLAN ID is part of the VLAN tag added to the packets by VLAN switches and routers. 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. VLAN ID 0 is used only for high priority frames, and 4095 is reserved.
All devices along a route must support the VLAN ID of the traffic along that route. Otherwise, the traffic will be discarded before reaching its destination. For example, if your computer is part of VLAN_100 and a co-worker on a different floor of your building is also on the same VLAN_100, you can communicate with each other over VLAN_100, only if all the switches and routers support VLANs and are configured to pass along VLAN_100 traffic properly. Otherwise, any traffic you send your co-worker will be blocked or not delivered.
VDOM
If VDOMs are enabled, each VLAN subinterface must belong to a VDOM. This rule also applies for physical interfaces.
Interface-related CLI commands require a VDOM to be specified, regardless of whether the FortiGate unit has VDOMs enabled.
VLAN subinterfaces on separate VDOMs cannot communicate directly with each other. In this situation, the VLAN traffic must exit the FortiGate unit and re-enter the unit again, passing through firewalls in both directions. This situation is the same for physical interfaces.
A VLAN subinterface can belong to a different VDOM than the physical interface it is part of. This is because the traffic on the VLAN is handled separately from the other traffic on that interface. This is one of the main strengths of VLANs.
The following procedure will add a VLAN subinterface called VLAN_100 to the FortiGate internal interface with a VLAN ID of 100. It will have an IP address and netmask of
172.100.1.1/255.255.255.0, and allow HTTPS, PING, and Telnet administrative access. Note that in the CLI, you must enter “set type vlan” before setting the vlanid, and that the allowaccess protocols are lower case.
To add a VLAN subinterface in NAT mode - web-based manager
1. If Current VDOM appears at the bottom left of the screen, select Global from the list of VDOMs.
3. Select Create New to add a VLAN subinterface. 4. Enter the following:
5. Select OK.
To view the new VLAN subinterface, select the expand arrow next to the parent physical interface (the internal interface). This will expand the display to show all VLAN subinterfaces on this physical interface. If there is no expand arrow displayed, there are no subinterfaces configured on that physical interface.
For each VLAN, the list displays the name of the VLAN, and, depending on column settings, its IP address, the Administrative access you selected for it, the VLAN ID number, and which VDOM it belongs to if VDOMs are enabled.
To add a VLAN subinterface in NAT mode - CLI config system interface
edit VLAN_100
set interface internal set type vlan
set vlanid 100
set ip 172.100.1.1 255.255.255.0 set allowaccess https ping telnet end