To supportdual VLAN tagging(often referred to as Q-in-Q or 802.1ad), a Juniper interface must be configured to process more than one 802.1Q header. In Junos OS, this is handled at the physical interface level ([edit interfaces ]).

According to Juniper Service Provider documents, two primary configuration statements enable this capability:

stacked-vlan-tagging (Option D):This is the traditional command used to enable an interface to accept frames with two VLAN tags. When this is enabled, the router expects an outer "service" tag and an inner "customer" tag. This is specifically used in provider edge scenarios where a service provider is tunneling multiple customer VLANs.

flexible-vlan-tagging (Option A):This is a more modern and versatile command. It allows the interface to support a mix of different encapsulation types across different logical units. For example, with flexible-vlan-tagging, you can have one logical unit (unit 10) doing standard single-tagging and another logical unit (unit 20) doing dual-tagging (vlan-tags outer X inner Y). This is the preferred method on newer hardware (like the MX Series) because it provides the highest level of configuration flexibility.

Vlan-tagging (Option C)only enables the interface to support a single 802.1Q tag, andgigether-options (Option B)contains physical-layer settings like auto-negotiation or flow control, which do not influence VLAN encapsulation. Therefore, A and D are the correct mechanisms for enabling dual-tag support.