When is a loopback interface automatically in the up state?

• A. As long as the router is functioning
• B. Only if a T1 line is configured
• C. Only if a VLAN is configured
• D. When a physical interface is up

Answer: (A)

Loopback interfaces are virtual inside the router, so they don’t depend on any physical link. They stay up as long as the router itself is operational and the loopback isn’t administratively shut down. This makes the loopback a reliable, always-present interface used for stable addressing and routing purposes. The other options point to physical or media-dependent connections (like a T1 line, a VLAN, or a physical interface), which aren’t required for the loopback to be up.