Perform one more iteration of the distance vector algorithm and test if there have been any distances that change because of this computation. If a router's distant vector changes, then it can broadcast to all the other router's that their $D$ labels are wrong (with this message combining with others if there are several wrong labels). This algorithm has message complexity $O(nm)$ for the exchange of $n$ distant vectors of size $O(n)$ over $m$ edges, plus $O(m)$ additional messages for a flooding error message. Thus, the total message complexity is $O(nm)$.