e the common modes of Ethernet have differing levels of duplex support: 10Base-T 10Base-T was originally designed without full-duplex support. Some implemen- tations of 10Base-T support full duplex, but most do not. 100Base-T 100Base-T has long supported full duplex, which has been the preferred method for connecting 100-Mbps links for as long as the technology has existed. How- ever, the default behavior of 100Base-T is usually half duplex, and it must be set to full duplex, if SO desired. Because of the lack of widespread full-duplex support on 10Base-T, and the typical default behavior of 100Base-T, when auto-negotiation falls through to the parallel detection phase (which only detects speed), the safest thing for the driver to do is to choose half-duplex mode for the link. When Auto-Negotiation Fails When auto-negotiation fails on 10/100 links, the most likely cause is that one side of the link has been set to 100/full, and the other side has been set to auto-negotiation. This results in one side being 100/full, and the other side being 100/half. Chapter 3: Auto-Negotiation Figure 3-1 shows a half-duplex link. In a half-duplex environment, the receiving (RX) line is monitored. If a frame is present on the RX link, no frames are sent until the RX line is clear. If a frame is received on the RX line while a frame is being sent on the transmitting (TX) line, a collision occurs. Collisions cause the collision error counter to be incremented-and the sending frame to be retransmitted-after a random back-off delay. Half Duplex Half Duplex Server If receiving a packet on the RX side of Switch the interface, do not transmit Ethernet interface Ethernet interface If a packet is received while one is being transmitted, this is considered a collision Figure 3-1. Half duplex Figure 3-2 shows a full-duplex link. In full-duplex operation, the RX line is monitored, and the TX line is always considered available. Collisions do not occur full-duplex mode because the RX and TX lines are completely independent. Full Duplex Full D