When autonegotiation 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 autonegotiation. This results in one side being 100/full and the other side being 100/half.
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. This may seem counterintuitive in a modern switched environment, but remember that Ethernet was originally designed to work over a single wire. Switches and twisted pair came along later.
Figure 3-1. Half duplex
Figure 3-2 shows a full-duplex link. In full-duplex operation, the RX line is not monitored, and the TX line is always considered available. Collisions do not occur in full-duplex mode because the RX and TX lines are completely independent.
Figure 3-2. Full duplex
When one side of the link is full duplex and the other side is half duplex, a large number of collisions will occur ...