In a brownfield deployment scenario, the deployment of the underlay network must be taken with manual configuration. The other configurations, such as subnet stretching, LAN automation, and automated underlay, can be done with the help of Cisco DNA Center.

A higher local preference on R3 for the relevant routes is the correct method to influence outbound traffic inside the autonomous system. Local preference is a well-known discretionary BGP attribute used within an AS to choose the preferred exit path. A higher local preference is preferred over a lower one, and the attribute is propagated to iBGP peers so routers in the AS make a consistent outbound decision. The requirement says all traffic originating from AS100 must pass through AS200 toward the NTP and DHCP server, and if the R3- to-R4 link fails, the path must move to the R2-to-R9 link. Applying local preference on R3 toward the routers that need to reach the data center establishes R3 as the primary exit while preserving the alternate path through R2 and R9. AS-path prepending is mainly used to influence inbound traffic from other autonomous systems, not the local AS exit choice. Redistributing IGP metrics into BGP is less deterministic and not the right policy control. Reference topics: BGP local preference, outbound traffic engineering, iBGP policy propagation, primary and backup path design.

https://www.cisco.com/c/en/us/support/docs/quality-of-service-qos/qos-policing/19645- policevsshape.html