You may want to double-check traffic is really going over client interface for all netperf tests.  If the test interface is using same //subnet// as main NIC, and main NIC comes up first, then kernel will not use client NIC for traffic.  This is part of how TCP stack operates and is not possible/easy to change.  Instead of using generic <code>vm.get_address()</code> it may be necessary to force alternate subnet, or hard-code something special for this test.