Using BGP path advertisements to implement routing policy. Consider the network below with interconnected provider networks and customer networks. Assume that provider network D only wants to carry traffic that originates in one of its customer networks (w, z) or is destined to one of its customer networks. It specifically does not want to forward datagrams between two other provider networks, since that transit traffic is neither sourced from, nor destined to, a customer network of D. 

Now suppose D receives the path Cy from network

C. To which other networks will network D advertise the path Cy?