1. A network comprising: a first cluster of first switching devices, each first switching device compatible with a software-defined networking (SDN) protocol; a second cluster of second switching devices within the first cluster, each second switching device compatible with a protocol for an open systems interconnection (OSI) model layer; wherein the first switching devices comprise a plurality of border switching devices located at a boundary between the first cluster and the second cluster, each border switching device also compatible with the protocol for the OSI model layer; wherein the first switching devices effect first multipathing through the network except through the second cluster, and the second switching devices effect second multipathing just through the second cluster of the network, and wherein each second switching device is directly connected to just a pair of the border switching devices, and just routes network traffic between the pair of the border switching devices through the second cluster of the network, such that the first switching devices and the second switching devices together effect end-to-end multipathing through both the first cluster and the second cluster of the network.