Unnumbered BGP interfaces are particularly useful in deployments where IPv4 prefixes are advertised through BGP over a section without any IPv4/IPv6 address configuration on links. As a result, the routing entries are also IPv4 for destination lookup and have IPv6 next hops for forwarding purposes.
In the following EVPN deployment from the OS10 Networking Lab (see Support and References for a hands-on-lab deploying this topology), we have deployed Unnumbered BGP between the leaf and spine nodes.
Unnumbered BGP sessions are established between point‐to‐point links between nodes and are not supported between loopback interfaces of neighboring nodes. The feature is supported in all types of supported interface types, such as physical interfaces, port‐channel interfaces, and VLAN interfaces.
Unnumbered interfaces do not have an IP address associated with them. The interface should be made routing-capable by providing no switchport command and enable IPv6 route advertisement for neighbor discovery.
The topology in the figure below is deploying EVPN with symmetric IRB, Type-5 routes with Unnumbered BGP. The EVPN aspect of the deployment is not discussed here.
Figure 3. Use case deployment