Hello
Root bridge will send BPDUs across all its DP ports. following links might be useful for you:
http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24062-146.html
https://supportforums.cisco.com/discussion/11061531/stp-root-port-vs-designated-port
Hope this helps
Best Regards,
Zaid