Consider the setup shown in Figure: LDP adjacency and session over unnumbered interface.
LSR A and LSR B have the following LDP identifiers respectively:
<LSR Id=A> : <label space id=0>
<LSR Id=B> : <label space id=0>
There are two P2P unnumbered interfaces between LSR A and LSR B. These interfaces are identified on each system with their unique local link identifier. In other words, the combination of {Router-ID, Local Link Identifier} uniquely identifies the interface in OSPF or IS-IS throughout the network.
A borrowed IP address is also assigned to the interface to be used as the source address of IP packets which need to be originated from the interface. The borrowed IP address defaults to the system loopback interface address, A and B respectively in this setup. The user can change the borrowed IP interface to any configured IP interface, loopback or not, by applying the following command:
config>router>if>unnumbered [<ip-int-name | ip-address>]
When the unnumbered interface is added into LDP, it has the following behavior.