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Configuring a Preferred Source Address for Unnumbered Ethernet or Demux Interfaces

When a loopback interface with multiple secondary IPv4 addresses is configured as the donor interface for an unnumbered Ethernet or demux interface, you can optionally specify any one of the loopback interface’s secondary addresses as the preferred source address for the unnumbered Ethernet or demux interface. This feature enables you to use an IP address other than the primary IP address on some of the unnumbered Ethernet or demux interfaces in your network.

To configure a secondary address on a loopback donor interface as the preferred source address for an unnumbered Ethernet or demux interface, include the preferred-source-address option in the unnumbered-address statement:




unnumbered-address interface-name <preferred-source-address address>;

You can include this statement at the following hierarchy levels:

[edit interfaces interface-name unit logical-unit-number family inet]
[edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number family inet]

The following considerations apply when you configure a preferred source address on an unnumbered Ethernet or demux interface:

The unnumbered-address statement currently supports the configuration of a preferred source address only for the IPv4 address family. Configuration of a preferred source address for families other than IPv4 is not currently supported.
If you do not specify the preferred source address, the router uses the default primary IP address of the donor interface.
You cannot delete an address on a donor loopback interface while it is being used as the preferred source address for an unnumbered Ethernet or demux interface.
The router uses the preferred source address, if configured for an unnumbered Ethernet or demux interface, in ARP requests and replies. ARP requests must match the preferred source address.

For a configuration example that illustrates this feature, see Example: Configuring the Preferred Source Address for an Unnumbered Ethernet Interface.

To display the preferred source address for an unnumbered Ethernet or demux interface, use the show interfaces operational mode command. For information about using this command, see the JUNOS Interfaces Command Reference.

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