Loopback interface
A virtual network interface on a router/switch that's always up as long as the device is alive — it isn't tied to any physical port that can go down. Used as a stable identity and source address for routing protocols and management.
Why: a physical interface goes down if the cable is pulled or the link dies; a loopback exists only logically and stays up regardless of physical status. Use case: router ID in OSPF/BGP (a stable identity that doesn't change when a port flaps), BGP source address (iBGP sessions are often peered against loopbacks so the session survives if one of several physical paths fails), and management/SSH (an address that's always reachable regardless of which port you reach the device via). Since it never goes down, it gives stability to everything that needs a reliable address. Distinct from the 127.0.0.1 loopback on a host (that's a related but separate OS concept). A small but fundamental design principle in routed networking. Related to BGP peering and OSPF area.