Hi,
In V17 and earlier firmware, if you had an interface which was only going to operate on a Vlan, you needed to configure the physical interface with an IP address and assign it to a Zone in order to be able to configure a Vlan and address details for the port.
In Australia, we have ISP's who provide connections as PPPOE but operate on a specific Vlan.
The downside of this is that the Control Center will display as an interface down, even though it actually isnt, or if you configure it as a WAN interface you will get some firewall issues.
In V18 the limitation has been removed, so you can create a Vlan without having to configure the Physical Port. But what you don not have on the Vlan port is the ability to change the MTU size, as this is tied to the physical port.
With the Physical port as being un-configured, it operates on the default MTU size of 1500, even though there are times that a smaller MTU size is required.
I had configured my V18 EAP2 initially to use the ISP connection on Vlan2 without configuring the physical interface. But I was experiencing PPPOE reconnects several times a day. Attempting to change the MTU size fails, as the interface is un-configured and resets to the default every time.
Configuring the interface as a DMZ port, assigning an IP address and setting the MTU size to 1492 has resolved the PPPOE connection issue, and there has been no reconnection in several days.
V18 firmware needs the ability to set the MTU size on physical interfaces, even if they are un-configured if they are going to be used for Vlan use only.