Metaheuristic approach for the clustered VRP

The clustered Vehicle Routing Problem is defined as follows. An unlimited number of identical vehicles is available in the depot, and have to deliver goods to n customers. The capacity of the vehicles is denoted by K and the demand of customer ci is denoted by Di for all i in {1, . . . , n}. A partition of customers into m clusters is also provided, and all the customers that belong to the same cluster have to be visited by the same vehicle (or equivalently in the same trip). The problem objective is to visit all the customers exactly once while minimising the total travelled distance.