Load balancing is performed in a network using flow-based routing. For example, upon detection of a big flow, one or more alternative paths from a source host to a destination host in the network may be discovered by probing the network and generating, for each of the one or more alternative paths, an association of the packet header information of the big flow to an alternative path discovered using results of probing the network. Upon congestion in a path currently being used by the big flow, an alternative path that is not congested is selected from the one or more discovered alternative paths. The packet header information of the big flow is altered using the generated association of the packet header information to the selected alternative path such that the big flow will be transmitted using the selected alternative path.