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Understanding the Loopback Interface

• 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 interface address never changes and is always up if the device is up.

  When you ping an individual network 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.

• 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.
• Packet filtering—Stateless firewall filters can be applied to the loopback address to filter packets originating from, or destined for, the Routing Engine.

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. The most commonly used loopback IP address is 127.0.0.1 for IPv4 and ::1 for IPv6. The standard domain name for the address is localhost. On the lo0.0 interface, it is useful to have the IP address 127.0.0.1 or ::1 (or both) configured, as certain processes such as NTP and MPLS ping use this default host address The 127.0.0.1/32 and ::1 addresses are martian IP addresses. Martian IP addresses are invalid for routing, so are never advertised by the Juniper Networks device.

In addition to the localhost 127.0.0.1 or ::1 address, it is important to configure at least one loopback interface address that is valid for routing and that is unique in your network infrastructure so that it can be used for device identification.

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.