Understanding the Loopback Interface

The loopback address (lo0) has several uses, depending on the particular Junos feature being configured. It can perform the following functions:

Device identification—The loopback interface is used to identify the device. While any interface address can be used to determine if the device is online, the loopback address is the preferred method. Whereas interfaces might be removed or addresses changed based on network topology changes, the loopback address never changes.
When you ping an individual interface address, the results do not always indicate the health of the device. For example, a subnet mismatch in the configuration of two endpoints on a point-to-point link makes the link appear to be inoperable. Pinging the interface to determine whether the device is online provides a misleading result. An interface might be unavailable because of a problem unrelated to the device's configuration or operation. See the Junos OS Administration Guide for Security Devices.
Routing information—The loopback address is used by protocols such as OSPF to determine protocol-specific properties for the device or network. Further, some commands such as ping mpls require a loopback address to function correctly. See Verifying an OSPF Configuration in the Junos OS Routing Protocols and Policies Configuration Guide for Security Devices and Example: Configuring LDP-Signaled LSPs in the Junos OS MPLS Configuration Guide for Security Devices.
Packet filtering—Stateless firewall filters can be applied to the loopback address to filter packets originating from, or destined for, the Routing Engine. See Stateless Firewall Filter Configuration Overview in the Junos OS Routing Protocols and Policies Configuration Guide for Security Devices.

The Internet Protocol (IP) specifies a loopback network with the (IPv4) address 127.0.0.0/8. Most IP implementations support a loopback interface (lo0) to represent the loopback facility. Any traffic that a computer program sends on the loopback network is addressed to the same computer. The most commonly used IP address on the loopback network is 127.0.0.1 for IPv4 and ::1 for IPv6. The standard domain name for the address is localhost.

The device also includes an internal loopback address (lo0.16384). The internal loopback address is a particular instance of the loopback address with the logical unit number 16384. Junos OS creates the loopback interface for the internal routing instance. This interface prevents any filter on lo0.0 from disrupting internal traffic.

Understanding the Loopback Interface

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