You have two MX Series devices connected together and configured for Layer 2 operations. The interface that connects the two routers must carry…

To transport both tagged and untagged Layer 2 traffic over a single physical link between two MX Series devices, the connecting interfaces must be configured as trunk ports. A trunk port is designed to carry traffic for multiple VLANs, which it does by using 802.1Q tags to identify the VLAN for each frame. To handle untagged traffic, the native-vlan-id option is configured on the trunk interface. Any untagged frames received on this interface are then associated with the specified native VLAN. This configuration effectively allows the single link to multiplex traffic from multiple tagged VLANs and one untagged VLAN.

A. Access ports are designed to carry traffic for a single VLAN and do not support multiple tagged VLANs. The native VLAN option is a feature of trunk ports.

C. An access port, even with a voice VLAN, is not designed to carry traffic for multiple general-purpose tagged VLANs, which is a primary function of a trunk port.

D. The voice VLAN option is a specialized feature, typically for access ports, to prioritize voice traffic. The standard mechanism for handling untagged traffic on a trunk is the native-vlan-id setting.

1. Juniper Networks TechLibrary, Junos OS, Layer 2 Switching and VLANs for Routing Devices. Configuring VLANs on Interfaces (ELS). This document explains the different interface modes. It states, "Trunk—Configure a trunk interface to accept packets with an 802.1Q tag... To accept untagged packets on a trunk interface, you must configure a native VLAN ID." This directly supports the use of a trunk port with a native VLAN.

2. Juniper Networks TechLibrary, Junos OS, Layer 2 Switching and VLANs for Routing Devices. Example: Configuring a Trunk Port to Accept Untagged Packets. This configuration example demonstrates the exact scenario. It shows setting the port-mode to trunk and then configuring the native-vlan-id on that same interface to process untagged packets. The commands shown are set interfaces ge-fpc/pic/port unit 0 family ethernet-switching port-mode trunk and set interfaces ge-fpc/pic/port unit 0 family ethernet-switching native-vlan-id .

3. Juniper Networks TechLibrary, Junos OS, Layer 2 Switching and VLANs for Routing Devices. Understanding Bridging and VLANs on Switches. This guide defines the interface types: "Access interfaces admit untagged packets... Trunk interfaces multiplex traffic from multiple VLANs and discriminate among them by their VLAN tags." This clarifies why an access port (Options A and C) is incorrect for carrying multiple tagged VLANs.