An integer identifier or tag ID for this S-VLAN.
When S-VLANs are enabled, the VLAN and S-VLAN ID valuse must be unique
among subinterfaces configured on the same underlying major VLAN
interface.
If the S-VLANs are enabled, then the usdVlanSubIfVlanUntagged field must
be disabled.
Cannot be changed if the subinterface has upper bindings.