The design uses BGP route reflectors to reduce the full-mesh IBGP requirement. Each cluster has one route reflector peering with two clients, giving eight client-to-reflector peerings across four clusters. The four route reflectors must also peer with each other in a full mesh, adding six peerings. This results in a total of fourteen IBGP peerings.

Border Node is part of the overlay.
https://www.cisco.com/c/en/us/td/docs/solutions/CVD/Campus/cisco-sda-design- guide.html#OverlayNetwork

The fabric management plane in a Cisco SD-Access architecture is responsible for managing the automation of device deployments and configurations within the fabric. It provides centralized control to simplify operations by automating tasks such as network device provisioning, configuration, and monitoring. This management plane enables a more streamlined and efficient deployment process across the SD-Access network, ensuring consistency and reducing manual configuration errors.