https://www.ciscopress.com/articles/article.asp?p=2995354&seqNum=2#:~:text=The%20Southbound%
20Interface,communicate%20to%20the%20networking%20devices.&text=The%20overall%20goal%20is%
20network,from%20being%20only%20a%20protocol.
The Southbound Interface
In a controller-based network architecture, the controller needs to communicate to the networking devices.

Explanation:

The root bridge is determined by the lowest bridge ID, which consists of the priority value and the MAC address. Because the priority values of all of the switches are not avalable, the MAC address is used to determine the root bridge. Because S3 has the lowest MAC address, S3 becomes the root bridge.