modifying tire sizing
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.