 Dual support vectors and margin degeneracy

A dual support vector has ${\alpha }_i>0$. Every such point has signed margin at most one, but a point exactly on the margin can have ${\alpha }_i=0$ in a degenerate solution. If ${\alpha }_i=0$, complementary slackness forces ${\xi }_i=0$, so the point is correctly classified with margin at least one.