I’m having some troubles with Bellman Ford Algorithm in the following exercise[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/G95WU.jpg

My guess is that at the end the algorithm should detect the negative cycle BD DF FG GB since B is decreasing at Vth (i.e. 7th) iteration. However changing the distance of the source does not make much sense to me. Is the source to be considered at fixed distance 0 or it can change as well as all the other vertices? Or is it correct to assume, as I did, that the algorithm will detect a negative cycle in the end? 
Thank you