There is disclosed a port unit and methods for testing a network. The port unit may include a traffic generator, traffic receiver, and a port processor. The traffic generator may include a memory that stores data defining a plurality of packet streams. The port processor may store label tracking data associating a plurality of label condition sets with one or more respective locations within the traffic generator memory. When a control packet associating a MPLS label with a specific label condition set is received via the traffic receiver, the port processor may store the MPLS label in one or more traffic generator memory locations associated with the specific label condition set in accordance with the stored label tracking data.