We introduce in this paper a bandwidth aggregation routing solution for multihoming sites. Our routing solution interconnects two distinct multihomed network sites (i.e. network sites that have two or more uplinks to the Internet) and routes local flows between these two network sites. It routes local flows dynamically through several outgoing network paths/links depending on the load (i.e. congestion level) on each path. If a network path/uplink becomes more congested, fewer local flows are routed through it. We detail two path load estimation strategies: one based on RTT measurements and the other based on throughput measurements, both implying passive network measurements. We performed a significant number of experiments in order to show that our multihoming solution performs better than an ECMP-based (i.e. Equal-Cost Multipath) solution in terms of total aggregated throughput and inter-flow fairness.