Auto-Negotiation

When first installed, a 100BaseTX Fast Ethernet interface negotiates with the destination device to which it is attached. The negotiation process occurs automatically, and involves each device notifying (advertising to) the other what connection capabilities it can support, including line speed and whether the circuit is half- or full-duplex. The devices determine what common set of capabilities both support and then, from that common set, select the connection that provides the greatest throughput. The common set of capabilities is typically referred to as the "highest common denominator." No connection is established if the two devices do not support at least one common line speed and duplex setting combination.

For example, a Fast Ethernet interface on a VNswitch module may support only a full-duplex circuit on either 10BaseT or 100BaseX cable. The device to which it is connected supports a full-duplex circuit on only 100BaseX cable. The two devices "agree" to use a full-duplex circuit at 100 Mb/s, since both support 100BaseX cable, which provides greater bandwidth.