1. A method for supporting efficient reconfiguration of an interconnection network having a pre-existing routing comprising: providing, at one or more computers, including one or more microprocessors, a plurality of switches, the plurality switches comprising one or more leaf switches, wherein each of the one or more leaf switches comprise a plurality of ports, and a plurality of end nodes, wherein the plurality of end nodes are interconnected via the one or more leaf switches; detecting, by a subnet manager, a reconfiguration triggering event, the reconfiguration triggering event comprising at least one of a changed network traffic pattern and a drop in Quality-of-Service (QOS); computing, by the subnet manager, a new routing for the interconnection network, wherein the computing by the subnet manager of the new routing for the interconnection network takes into consideration the pre-existing routing and selects the new routing for the interconnection network that is closest to the pre-existing routing; and reconfiguring the interconnection network according to the new routing for the interconnection network; wherein the computing by the subnet manager of the new routing for the interconnection network comprises: preserving the pre-existing port selections for the plurality of end nodes, so long as said preserving the pre-existing port selections for the plurality of end nodes does not conflict with QOS requirements and so long as said preserving the pre-existing port selections for the plurality of end nodes does not conflict current traffic patterns.