Produces cycloisomaltooligosaccharide from dextran containing 7, 8 or 9 glucose units. The enzyme is specific for (1->6)-alpha-D-glucans (dextrans) and, without activity toward (1->4)-alpha-D-glucans, such as amylose. It also has no activity on oligosaccharides, such as amylopectin and pullulan, containing (1->6)-alpha-D-glucosidic linkages at branch points. cyclizes part of a (1->6)-alpha-D-glucan chain by formation of a (1->6)-alpha-D-glucosidic bond. Optimum pH is 5.5. Monomer. Belongs to the glycosyl hydrolase 66 family.