A method for measuring and determining the duplex modes of a network interface. The method assumes the network interface to be operating in a half-duplex mode until the bandwidth utilization reaches a threshold. When the threshold is reached, the method checks the traffic collision in the interface. If there is no collision, then the duplex mode is determined to be full-duplex. If there is collision, then the duplex mode is determined to be half-duplex and an alarm is set off. In another embodiment, the interface type is determined through SNMP. If the interface is a WAN interface, then the interface is determined to be full-duplex.