The IP unnumbered interfaces feature allows the use of the same IPv4 address on multiple interfaces, thus conserving IP addresses. The configuration of interfaces as unnumbered allows the enabling of IP processing on interfaces without assigning an IP address to them.
To configure an unnumbered interface, specify a donor interface from which the system can borrow an IP address. Then, configure a static-interface route to forward packets through the unnumbered interface.
Only data plane interfaces and virtual interfaces (vifs) can be configured as unnumbered interfaces. For IP packets that originate from routers, the unnumbered interfaces use IPv4 addresses from donor interfaces as preferred source addresses. The donor interfaces may be either data plane or loopback interfaces.