My husband had a Veterinarian Practice for over 35 years and just retired 2 years ago. We only administered the Bordatella once a year. The company that makes this advised that was all that is needed. The only circumstances in which is was administered at 6 mos. was if the dog was constantly exposed to other dogs for long durations. Hope this helps.
Just an added note: My husband filled in for several vets here in Florida (we're from Illinois) and he does believe that some of them give many unnecessary vaccines.