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Loopback Interface Configuration

You (a system administrator, network administrator, or end user) can use this procedure to configure the loopback interface on your device.

Configure the Loopback Interface

When specifying the loopback address on a device, do not include a destination prefix. Also, in most cases, specify a loopback address only on unit 0 and no others.


For Layer 3 virtual private networks (VPNs), you can configure multiple logical units for the loopback interface. This allows you to configure a logical loopback interface for each virtual routing and forwarding (VRF) routing instance.

For some applications, such as SSL for Junos XML protocol, at least one address for the interface lo0.0 must be

You can configure loopback interfaces using a host (recommended), a subnetwork address for both inet and inet6 address families, or an ISO network entity title (NET) address for the iso address family. Many protocols require a loopback address as their source address. Configuring a loopback address as a donor interface for unnumbered interfaces enables these protocols to run on unnumbered interfaces.

In some cases, the loopback interface can also be the router identifier (router ID). If the router ID is not explicitly configured, the device determines its router ID as shown in the following table:

Table 1: Default Router ID
If the loopback interface is: Then the default router ID is:
Configured The loopback interface
Not configured The lowest IP address of any interface in operational state up

In both cases, the router ID changes when the operational state of the interface changes. Therefore, we recommend configuring the address on a stable loopback interface.

If you configure more than one address on the loopback interface, we recommend that you configure one to be the primary address. The device selects the primary address as the router ID when the router ID is not configured. The device also uses the primary address as the default source address for traffic sourced from the loopback interface by the Routing Engine.

To configure the physical loopback interface (lo0), include the following statements at the [edit interfaces] hierarchy level:

You can configure one or more addresses on the loopback interface. You can configure more than just unit 0 for lo0, but you must place each additional unit in a separate routing instance.

Example: Configure Two Addresses on the Loopback Interface with Host Routes

In the following example, the user configures two addresses on the loopback interface with host routes: