What is the purpose of a loopback interface on a router or switch?

• A. It is used to connect to the WAN.
• B. Loopback provides a stable, always-up virtual interface for management, testing, and addressability independent of physical ports.
• C. It replaces all physical ports in the device.
• D. It is used only for IPv6 traffic.

Answer: (B)

A loopback interface is a virtual, always-on path inside a router or switch. Because it isn’t tied to any physical port, it stays up regardless of the status of the device’s other interfaces. This stability makes it ideal for reliable management access, testing, and addressing—the device can always be reached at a known loopback address, and many routing protocols use the loopback address as a stable router identifier.

That’s why this option is the best: it describes a stable, always-up virtual interface that serves management, testing, and consistent addressability independent of physical ports. It isn’t used to connect to the WAN, it doesn’t replace all physical ports, and it isn’t limited to IPv6 traffic.