From Chengdu over Aba, Garze to Derge. Especially the last part after Garze is fantastic as you cycle along the Chola Mountains and finally over them.
There are many other option for the start. The one that I have choosen is certainly not the shortest... Very nice is the route over the Balang Shan pass and Danba, see
here.
As the whole route is outside of the TAR it is mostly possible without problems.