1-11 Logical Interface ConfigurationWhen configuring logical interfaces, go to these sections for information you are interested in:z Loopback Interfacez Null Interfacez Displaying and Maintaining Logical InterfacesLoopback InterfaceIntroduction to Loopback InterfaceA loopback interface is a software-only virtual interface. The physical layer state and link layerprotocols of a loopback interface are always up unless the loopback interface is manually shut down.A loopback interface can be configured with an IP address. For the purpose of saving IP addressresources, the IP address is automatically configured with a 32-bit mask. Routing protocols can beenabled on a loopback interface, and a loopback interface is capable of sending and receiving routingprotocol packets.Loopback interfaces are widely used. For example, you can configure a loopback interface address asthe source address of all the IP packets that the device generates. Because loopback interfaceaddresses are stable unicast addresses, they are usually used as device identifications. Therefore,when you configure a rule on an authentication or security server to permit or deny packets generatedby a device, you can streamline the rule by configuring it to permit or deny packets carrying theloopback interface address identifying the device. Note that, when a loopback interface is used forsource address binding(that is, assigning an IP address to this loopback interface), make sure that theroute from the loopback interface to the peer is reachable; all data packets sent to the loopbackinterface are considered as packets sent to the device itself, so the device does not forward thesepackets.Because a loopback interface is always up, it can be used for some other special purposes. Forexample, if no router ID is configured for a dynamic routing protocol, the highest loopback interface IPaddress is selected as the router ID. For another example, to avoid BGP sessions being interrupted byphysical port failure, you can use a loopback interface as the source interface of BGP packets.However, you must ensure that the IP address of the loopback interface on the BGP peer is reachable.If EBGP connection is involved, you need to allow the establishment of EBGP connections tonon-directly-connected peers.Configuring a Loopback InterfaceFollow these steps to configure a loopback interface:To do… Use the command… RemarksEnter system view system-view —
