What specific command is used to encapsulate sub-interfaces for Q-in-Q tunneling?

The command used to encapsulate sub-interfaces for Q-in-Q tunneling is "encapsulation dot1q." This command is essential when configuring VLANs on sub-interfaces because it specifies the IEEE 802.1Q encapsulation method, which is necessary for recognizing and forwarding tagged frames in a network that uses VLANs.

In Q-in-Q tunneling, multiple VLAN tags can be added to frames as they traverse between switches, which allows service providers to maintain separate customer VLANs while forwarding the traffic over their infrastructure. By applying this encapsulation command to a sub-interface, you instruct the device to treat the traffic according to the VLAN tagging scheme specified. This is particularly crucial for ensuring that the encapsulated frames reach their destination with the correct identifiers that facilitate proper routing and processing by other devices in the network.

Other commands, such as "no shutdown," are used to enable an interface but do not relate directly to the encapsulation of VLANs. Similarly, "switchport trunk" is a command used in switch configurations for trunk ports in Layer 2 networks but does not apply specifically to configuring sub-interfaces for tunneling. Therefore, the specificity and correctness of the "encapsulation dot1q" command make it the appropriate choice for Q