To configure unnumbered IP on an interface, you must specify the interface from which to borrow the IP address. The borrowed interface must already exist and have an IP address configured on it.

The status of an unnumbered IP interface is down whenever the borrowed interface is down. Therefore, it is recommended to borrow the IP address from an interface that is always up, such as the Loopback interface.

Routes discovered on an unnumbered interface by the RIP and OSPF routing protocols are displayed as via routes in the routing table. The next hop is listed as via the IP unnumbered interface instead of the source address of the routing update.