Configuring an Unnumbered Interface
When you need to conserve IP addresses, you can configure unnumbered interfaces. Setting up an unnumbered interface enables IP processing on the interface without assigning an explicit IP address to the interface. For IPv6, in which conserving addresses is not a major concern, you can configure unnumbered interfaces to share the same subnet across multiple interfaces. IPv6 unnumbered interfaces are only supported on Ethernet interfaces. The statements you use to configure an unnumbered interface depend on the type of interface you are configuring: a point-to-point interface or an Ethernet interface:
Configuring an Unnumbered Point-to-Point Interface
To configure an unnumbered point-to-point interface, configure the protocol family, but do not include the address statement:
You can include this statement at the following hierarchy levels:
- [edit interfaces interface-name unit logical-unit-number]
- [edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number]
 | Note: For interfaces with PPP encapsulation, you can configure an unnumbered interface by including the unnumbered-interface statement in the configuration. For more information, see Configuring IPCP Options. |
When configuring unnumbered interfaces, you must ensure that a source address is configured on some interface in the router. This address is the default address. We recommend that you do this by assigning an address to the loopback interface (lo0), as described in Configuring the Loopback Interface. If you configure an address (other than a martian) on the lo0 interface, that address is always the default address, which is preferable because the loopback interface is independent of any physical interfaces and therefore is always accessible.
Example: Configuring an Unnumbered Point-to-Point Interface
Configure an unnumbered point-to-point interface:
Configuring an Unnumbered Ethernet or Demux Interface
To configure an unnumbered Ethernet or demultiplexing interface, include the unnumbered-address statement in the configuration:
You can include this statement at the following hierarchy levels:
- [edit interfaces interface-name unit logical-unit-number family family]
- [edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number family family]
For dynamic profiles, include the unnumbered-address statement at the following hierarchy levels:
- [edit dynamic-profiles profile-name interfaces interface-name unit logical-unit-number family family]
- [edit dynamic-profiles profile-name interfaces demux0 unit logical-unit-number family family]
The unnumbered-address statement currently supports configuration of unnumbered demux interfaces only for the IPv4 address family. You can configure unnumbered Ethernet interfaces for both IPv4 and IPv6 address families.
The interface that you configure to be unnumbered borrows an assigned IP address from another interface, and is referred to as the borrower interface. The interface from which the IP address is borrowed is referred to as the donor interface. In the unnumbered-address statement, interface-name specifies the donor interface. For an unnumbered Ethernet interface, the donor interface can be an Ethernet, ATM, SONET, or loopback interface that has a logical unit number and configured IP address and is not itself an unnumbered interface. For an unnumbered IP demultiplexing interface, the donor interface can be an Ethernet or loopback interface that has a logical unit number and configured IP address and is not itself an unnumbered interface. In addition, for either Ethernet or demux, the donor interface and the borrower interface must be members of the same routing instance and the same logical system.
When you configure an unnumbered Ethernet or demux interface, the IP address of the donor interface becomes the source address in packets generated by the unnumbered interface.
You can configure a host route that points to an unnumbered Ethernet or demux interface. For information about host routes, see the Junos OS MPLS Applications Configuration Guide.
For more information, see the following sections:
- Configuring a Preferred Source Address for Unnumbered Ethernet or Demux Interfaces
- Configuring Static Routes on Unnumbered Ethernet Interfaces
- Restrictions for Configuring Unnumbered Ethernet Interfaces
- Example: Configuring an Unnumbered Ethernet Interface
- Example: Configuring the Preferred Source Address for an Unnumbered Ethernet Interface
- Example: Configuring an Unnumbered Ethernet Interface as the Next Hop for a Static Route
For additional information about dynamic-profiles, see Dynamic Profiles Overview.
Configuring a Preferred Source Address for Unnumbered Ethernet or Demux Interfaces
When a loopback interface with multiple secondary IP 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:
You can include this statement at the following hierarchy levels:
- [edit interfaces interface-name unit logical-unit-number family family]
- [edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number family family]
- [edit dynamic-profiles profile-nameinterfaces interface-name unit logical-unit-number family family]
- [edit dynamic-profiles profile-name interfaces demux0 unit logical-unit-number family family]
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 for demux interfaces, and for IPv4 and IPv6 address families for Ethernet interfaces.
- 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 OS Operational Mode Commands.
Configuring Static Routes on Unnumbered Ethernet Interfaces
You can configure static routes on an unnumbered Ethernet interface. To do so, you use the qualified-next-hop statement to specify the unnumbered Ethernet interface as the next-hop interface for a configured static route. This feature enables you to specify independent preferences and metrics for static routes on a next-hop basis.
For a configuration example that illustrates this feature, see Example: Configuring an Unnumbered Ethernet Interface as the Next Hop for a Static Route.
For information about how to specify an independent preference for a static route, see the Junos OS Routing Protocols Configuration Guide.
Restrictions for Configuring Unnumbered Ethernet Interfaces
The following restrictions apply when you configure unnumbered Ethernet interfaces:
- The unnumbered-address statement currently supports the configuration of unnumbered Ethernet interfaces for IPv4 and IPv6 address families.
- You cannot assign an IP address to an Ethernet interface that is already configured as an unnumbered interface.
- The donor interface for an unnumbered Ethernet interface must have one or more configured IP addresses.
- The donor interface for an unnumbered Ethernet interfaced cannot be configured as unnumbered.
- An unnumbered Ethernet interface does not support configuration of the following address statement options: arp, broadcast, primary, preferred, and vrrp-group. For information about these options, see Configuring the Interface Address.
- Running IGMP and PIM are supported only on unnumbered Ethernet interfaces that directly face the host and have no downstream PIM neighbors. IGMP and PIM are not supported on unnumbered Ethernet interfaces that act as upstream interfaces in a PIM topology.
- Running OSPF and IS-IS on unnumbered Ethernet interfaces is not supported. However, you can run OSPF over unnumbered Ethernet interfaces configured as a Point-to-Point connection.
Example: Configuring an Unnumbered Ethernet Interface
In this example, ge-1/0/0 is the unnumbered interface and ge-0/0/0 is the donor interface from which ge-1/0/0 “borrows” an IP address.
Example: Configuring the Preferred Source Address for an Unnumbered Ethernet Interface
In this example, loopback interface lo0 is the donor interface from which unnumbered Ethernet interface ge-4/0/0 “borrows” an IP address. The example also configures one of the loopback interface’s secondary addresses, 3.3.3.1, as the preferred source address for the unnumbered Ethernet interface.
Example: Configuring an Unnumbered Ethernet Interface as the Next Hop for a Static Route
In this example, ge-0/0/0 is the unnumbered interface and a loopback interface, lo0, is the donor interface from which ge-0/0/0 “borrows” an IP address. The example also configures a static route to 7.7.7.1/32 with a next hop through unnumbered interface ge-0/0/0.0.
