From 8.3R3 onwards and 9.0R1, a new option "Default VLAN ID" has been added under "Advanced Port" section of Internal/External/Management port configuration section under System > Network settings as shown below:
A Default VLAN ID configured here will cause the PCS to tag "ALL" the untagged egress packets on that interface with the VLAN ID specified in the setting. One of the major advantages of the feature is the ability for authentication traffic to be tagged by the PCS before egressing out the PCS interface. For example, authentication traffic from PCS going out over the Internal (LAN) interface can be tagged with a VLAN ID. In previous releases, untagged authentication traffic had to be handled at the switch connecting to the corresponding PCS port. Please note
that setting a VLAN ID will cause the PCS port to listen on VLAN tagged traffic as well. Ensure that the switchport connecting to the corresponding PCS port forwards same VLAN ID tagged traffic to PCS, else connectivity/traffic flow to the corresponding PCS port will break.