IP devices come with an internal IP address of 127.0.0.1 that is for use internal to the device. The loopback interface is used as an IP address holder that is independent of any other interface. Configuring a loopback interface creates a logical internal IP interface for a router that can be used to identify the router to other routers. This use of the loopback as an identification will be discussed in Chapter 8. As you will see in Chapter 8 and beyond, when a second IP address is configured on the lo0 ( Loopback0 ) interface, it will serve as a stable identifier for the router to use in communications protocols. If a router uses an IP address of an interface to identify itself to other routers, other routers could lose communications if that interface failed. This is because another device cannot talk to a down interface. The lo0 interface, however, is always up as long as the router is up. The addressing for the loopback interface is configured just like it is on other interfaces ”the family type is assigned, and then the address is configured. Even though there is no physical presence, the interface has both a physical and unit hierarchy configuration to maintain configuration consistency. The following is a configured lo0 interface with the host IP address 192.168.5.2 : [edit interfaces lo0] lab@chicago# show unit 0 { family inet { address 192.168.5.2/32; } } [edit interfaces lo0] lab@chicago# When a show interfaces command is used, the loopback interface information is displayed just as that of a typical network interface is, with the physical and logical unit sections separated, as shown below. lab@chicago> show interfaces lo0 Physical interface: lo0, Enabled, Physical link is Up Interface index: 5, SNMP ifIndex: 7 Type: Loopback, MTU: Unlimited Device flags : Present Running Loopback Interface flags: SNMP-Traps Link flags : None Input packets : 10 Output packets: 10 Logical interface lo0.0 (Index 3) (SNMP ifIndex 25) Flags: SNMP-Traps Encapsulation: Unspecified Protocol inet, MTU: Unlimited, Flags: None Addresses, Flags: Is-Default Is-Primary Local: 192.168.5.2 The loopback is an internal interface kept by the router as an identifier. As long as the router is functional, the loopback will be functional, allowing the router to possess an IP address for identification that will not be lost if an interface or link fails. |