15Loopback and null interface configurationThis chapter includes these sections:• Loopback interface• Null interface• Displaying and maintaining loopback and null interfacesLoopback interfaceIntroduction to loopback interfaceA loopback interface is a software-only virtual interface. It delivers the following benefits.• The physical layer state and link layer protocols of a loopback interface are always up unless theloopback interface is manually shut down.• You can assign a loopback interface an IP address with an all-F mask to save 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 not 128-bit,the system automatically changes the mask into a 128-bit mask.• 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 theswitch 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 switch, you can simplify the rule byconfiguring it to permit or deny packets carrying the loopback interface address identifying theswitch. Note that, when you use a loopback interface address as the source address of IP packets,make sure that the route from the loopback interface to the peer is reachable by performing routingconfiguration. All data packets sent to the loopback interface are considered as packets sent to theswitch itself, so the switch does not forward these packets.Because of the benefits mentioned above, loopback interfaces are widely used. You can configure aloopback interface address as the source address of the IP packets that the device generates. Becauseloopback interface addresses are stable unicast addresses, they are usually used as deviceidentifications. Therefore, when you configure a rule on an authentication or security server to permit ordeny packets generated by a device, you can simplify the rule by configuring it to permit or deny packetscarrying the loopback interface address identifying the device. Note that, when you use a loopbackinterface address as the source address of IP packets, make sure that the route from the loopbackinterface to the peer is reachable by performing routing configuration. All data packets sent to theloopback interface are considered as packets sent to the device itself, so the device does not forwardthese packets.
