Switching

[edit interfaces ge-0/0/1]
user@router# show
unit100 {
vlan-id 100;
familyinet {
address /24;
}
}
[edit interfaces ge-0/0/1]
user@router# commit check
[edit interfaces ge-0/0/1]
'unit 100'
Only unit0 is valid for this encapsulation
error: configuration check-out failed

Referring to the above, you are unable to commit the configuration for the ge-0/0/1 interface.

What must you do to commit the configuration?

A. You must set the vlan-tagging parameter under the [edit interfaces ge-0/0/1] hierarchy.

In the example above, the answer says we need to be in the edit interfaces ge-0/0/1 hierarchy.

I was under the impression we are in that hierarchey, from the edample.

Any help on this.

What the answer is saying is to enable vlan-tagging on the ge-0/0/1 interface.  To enable vlan-tagging you need to navigate to the [edit interfaces ge-0/0/1] hierachy or you can do this from the top level as well i.e. set interfaces ge-0/0/1 unit vlan-tagging

OK, but I thought it was already at the hierarchy.

Yes you are under the heirarchy, but the option vlan-tagging was not configured. So this opotion is configured at the physical interface heirarchy as shown

 You must set the vlan-tagging parameter under the [edit interfaces ge-0/0/1] 

any configuration under the unit numbers would represent logical properties

Without eabling vlan-tagging, the only logical unit number you can configure is unit 0. Enabling vlan-tagging allows you to configure unit 0-4095 if the switch allows it.

set interfaces ge-0/0/1 vlan-tagging

BTW you will also get this error if you try to configure family ethernet-switching under any unit number other than 0