1-11 Loopback Interface and Null InterfaceConfigurationWhen configuring loopback interfaces and null interfaces, go to these sections for information you areinterested in:z Loopback Interfacez Null Interfacez Displaying and Maintaining Loopback and Null InterfacesLoopback InterfaceIntroduction to Loopback InterfaceA loopback interface is a software-only virtual interface. It delivers the following benefits:z The physical layer state and link layer protocols of a loopback interface are always up unless theloopback interface is manually shut down.z You can assign a loopback interface an IP address with an all-F mask, thus saving the IP addressresources. When you assign an IPv4 address whose mask is not 32-bit, the system automaticallychanges the mask into a 32-bit mask. When you assign an IPv6 address whose mask is not128-bit, the system automatically changes the mask into a 128-bit mask.z You can enable routing protocols on a loopback interface, and a loopback interface can send andreceive routing protocol packets.Because of the benefits mentioned above, loopback interfaces are widely used in the followingscenarios:You can configure a loopback interface address as the source address of the IP packets that thedevice generates. Because loopback interface addresses are stable unicast addresses, they areusually used as device identifications. Therefore, when you configure a rule on an authentication orsecurity server to permit or deny packets generated by a device, you can streamline the rule byconfiguring it to permit or deny packets carrying the loopback interface address identifying the device.Note that, when you use a loopback interface address as the source address of IP packets, make surethat the route from the loopback interface to the peer is reachable by performing routing configuration.All data packets sent to the loopback interface are considered as packets sent to the device itself, sothe device does not forward these packets.Configuring a Loopback InterfaceFollow these steps to configure a loopback interface:
