Loopback Interface

Every IP-capable network device has an interface called a loopback interface. A loopback interface can be found in PCs, switches, and routers. A loopback interface is an address that when pinged or routed to is processed internally by the device and not tied to any physical interface. It is a virtual interface when configured with an IP address that is always in an up/up (administratively up/operationally up) state. It can be used for management purposes, for testing purposes, and assigned as the RID for dynamic routing protocols. For testing the loopback address is used to test local processes and applications within the system and testing the local TCP/IP stack. For management the loopback interface IP can be accessed via SSH at any time as long as there is a valid route in the routing table. It can also be used as the RID that can be used for dynamic routing protocols like OSPF. The reason why the loopback interface would be used for management and RID IP addresses is because it is not connected to any interface on the device. Since it is a logical interface, it is basically always on and free from any type of failure as long as the device has power. So if any physical port fails, dynamic routes will still have a stable RID, and network admins can still manage the device despite the failure. OSPF and other protocols use RID to identify routers, so a stable IP address like a loopback address ensures the RID never changes, preventing the entire OSPF process from restarting back to zero. On PCs the loopback address is typically 127.0.0.1 and can also be called “localhost.” On other devices like routers and switches, the loopback interface can be assigned any IP address manually.