The main purpose of the Cisco SD-Access underlay design is to provide a scalable and optimized foundation for network traffic routing and load balancing. The underlay network is the physical infrastructure that supports the SD-Access fabric and allows efficient transport of data across different parts of the network. It helps with optimizing traffic flow, ensuring that data is routed effectively and with low latency, which is critical for supporting the overall network design.

https://www.cisco.com/c/en/us/td/docs/routers/sdwan/configuration/bridging-routing- segmentation-qos/vedge/bridging-routing-segmentation-qos-book/multicast-overlay-routing.html Replicators For efficient use of WAN bandwidth, strategic vEdge routers can be deployed and configured as replicators throughout the overlay network. Replicators mitigate the requirement for an ingress router to replicate a multicast stream once for each receiver.

In Cisco Catalyst SD-WAN, OMP (Overlay Management Protocol) by default enables a full mesh topology between all WAN Edge routers if no policy is defined, allowing direct communication among all sites.