This article is about the Class III road numbered C21 maintained by DfI Roads.
For other roads numbered C21, see C21
The C21 starts on a TOTSO on the B12 in Crumlin, following Lurgan Road out of town. The road initially heads west before turning south, meeting the C22 along the way. Heading south, the road is fairly straight with the odd bend here and there. The road terminates on a slightly scattered crossroads west of Glenavy with the B12 to the left and continuing straight on and the B156 on the right.