Abstract: A network testing system for a device under test (DUT) uses a test tool to generate test packets provided to the DUT as source traffic and to receive the test packets back from the DUT as destination traffic. Before the test packets are returned from the DUT to the test tool, an external router receives the test packets from the DUT and forwards them back to the DUT. The DUT has a first pair of DUT interfaces coupled to the test tool and a second pair of DUT interfaces coupled to the external router. The DUT has a first virtual router with a first routing table configured to direct test packets received on one of the interfaces in the first pair of DUT interfaces to the external router. The DUT has a second virtual router with a second routing table configured to direct test packets received on one of the interfaces in the second pair of DUT interfaces to the test tool. Thus, test packets output by the test tool traverse the DUT more than once before returning to the test tool.