what is VLAN ID 0 ? why cannot use VLAN ID 1?

[YasushiNarita]

Fortigate 90G FirmVer7.4.5

Default internal is VLAN ID 0.

I tried to change this VLAN ID 1,but errore poped up something like "cannont use VLAN ID 1..."

I want to use VLAN ID 1 ,because I want to fortigate deliver VLAN ID 1,100,200 by trunk-port to cisco L2switch.

How can i change VLAN ID 1 of fortigate? 

or it is not nessesary to define VLAN ID 1?

By the way, what means internal vlan id 0 is? 

[AEK]

You need to create 3 VLAN interfaces, with VLAN 1, 100 and 200.

You do it under menu Network > Interface, click "New", then select type VLAN and the appropriate ID. The 3 interfaces must be created on the physical interface that is connected to your Cisco switch as trunk interface.

[Toshi_Esumi]

https://docs.fortinet.com/document/fortigate/7.6.2/fortigate-7000f-administration-guide/729353/vlan-...

Toshi

[AEK]

8 years in FGT and I don't know this!

I grew up in ignorance.

[Toshi_Esumi]

I didn't know that either until last year when I was investigating F-series "VLAN switch"'s "trunk" port issue. Probably was not written anywhere clearly until recently.

Toshi

[YasushiNarita]

I've read this,as well.
I found this test below.

1. if I use VLAN switch  →cannot create VLAN ID 1.

2. if I use VLAN            →can create VLAN ID 1,

                                 though I'm worried about reserved VLAN ID document issue,

                                 which is written "Fortigate do not work as exepected."

I should not use VLAN ID 1,anyway...??

What do you think about it?
If I cannot use VLAN ID 1,it would be occuring Cisco vlan-connection problem,maybe..

[Toshi_Esumi]

If it says "don't use/reserved", don't use. That's FTNT's official message.
In the last VLAN switch thread, that person told us it could be configured but just didn't work.
And I'm not sure if Cisco's VLAN1 can be tagged at any interfaces. Shouldn't cause any issues connecting to FGTs as long as it's untagged (native VLAN).

Toshi