[ns] how to control CBR packets transmitted on different route

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 hi,
    supposed that there are two routes betwen node a and d, one through b ,and the other through c. that is, the first route is a->b->d, and the send route is a->c->d. 
    how can i send the different CBR packet on the differnt route. for example, i want send packets whose packet_id=1,3,5,7,...on the first route, and the packets whose packet_id=2,4,6,...,on the sencond route.
    thanks your help.
 
                                                                 hujie