 (Mbps). Common Ethernet speeds include 10 Mbps, 100 Mbps, and 1,000 Mbps. 1,000 Mbps Ethernet is also referred to as Gigabit Ethernet. Duplex Duplex refers to how data flows on the interface. On a half-duplex interface, data can only be transmitted or received at any given time. A conversation on a two-way radio is usually half-duplex-each person must push a button to talk, and, while talking, that person cannot listen. A full-duplex interface, on the other hand, can send and receive data simultaneously. A conversation on a telephone is full duplex. How Auto-Negotiation Works First, let's cover what auto-negotiation does not do: when auto-negotiation is enabled on a port, it does not automatically determine the configuration of the port on the other side of the Ethernet cable and then match it. This is a common miscon- ception that often leads to problems. Auto-negotiation is a protocol, and as with any protocol, it only works if it's running on both sides of the link. In other words, if one side of a link is running auto- negotiation, and the other side of the link is not, auto-negotiation cannot determine the speed and duplex configuration of the other side. If auto-negotiation is running on the other side of the link, the two devices decide together on the best speed and duplex mode. Each interface advertises the speeds and duplex modes at which it can operate, and the best match is selected (higher speeds and full duplex are preferred). The confusion exists primarily because auto-negotiation always seems to work. This is because of a feature called parallel detection, which kicks in when the auto- negotiation process fails to find auto-negotiation running on the other end of the link. Parallel detection works by sending the signal being received to the local 10Base-T, 100Base-TX, and 100Base-T4 drivers. If any one of these drivers detects the signal, the interface is set to that speed. Parallel detection determines only the link speed, not the supported duplex modes. This is an important consideration becaus