modifying tire sizing I use Schawlbe Durano Plus 700X23C on Bontrager TLB 622 X17 ERD rim and  want to change to a thinner tire as a 21C
Can I make this tire change on the same rim?
If this is possible wich tire brand?
TKS
If 17mm is the internal width, 21mm tires are probably a bad idea. According to Sheldon Browns (admittedly conservative) chart, 25mm is the lowest recomended width on 17mm rims. You could also probably contact Bontrager and ask for this specific rim though. 

As an aside, I'm not totally sure why you'd want to use 21mm tires. Barely anyone makes them anymore, they're less comfortable and have higher rolling resistance.